View Full Version : Piston: Long vs. Short
I've been doing some research into long versus short stroke piston designs for the AR pattern. It seems of my collection of pros/cons, the long stroke piston design is the better design, yet the long stroke is not the choice among the top major manufacturers (HK, Sig, Colt, LWRC, FN SCAR, others).
The short stroke popularity among the top manufacturers has me asking what am I missing from my pro/con list?
One advantage I have as a civilian is that I get to decide what is best for me and my situation. Perhaps it is just a matter of logistics; the short stroke manufacturers were given logistical military requirements that differ from my home logistics?
Can someone please enlighten me or add to my list?
21703
Thanks,
Jake
BRB... Getting my popcorn ready for this one.
mkmckinley
11-28-13, 23:49
Too much internet research not enough experience. Get a high quality DI AR, a bunch of ammo and some training. Six months from now you'll feel silly for getting wrapped up in the whole piston AR thing.
The SCAR isn't a AR derivative. It's probably more derived from the AR18 with a good mixing of G36 and some F2000 design elements too.
Its short stoke gas piston hits the bolt carrier dead center of the the carrier guide rails, eliminating carrier tilt. The charging handle is a simple steel piece with overmolded plastic cover inserted into either side of the bolt carrier, providing ambidextrous use.
H
I'd much rather have long stroke. Short strokes break things.
I'm a really big DI guy but if I had to run a piston and the only one I'm remotely interested in is the pws stuff. And they are the best pistons I've ever seen
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I'd much rather have long stroke. Short strokes break things.
FALs aren't really known for having a short service life.
H
FALs aren't really known for having a short service life.
H
The tilting bolt helps in that regard.
Can someone please enlighten me or add to my list?
21703
Thanks,
Jake
Most of these pros & cons are actually the other way round. :) Other than that, good that you are doing research. :)
- Forget the "parts are hitting..."
- longstroke pistons have inevitably more reciprocating mass -> """more""" recoil
- carrier tilt has nothing to do with the type of piston
- functioning dirty has nothing to do with the type of piston, but long strokes do tend to get dirtier faster.
- ability cycle "poor ammo" has nothing to do with the type of piston
- short strokes stay much cleaner and thus are easier to clean
- no real difference in weight (as in terms of balance or lugging it around)
- accuracy is not related to piston type
The tilting bolt helps in that regard.
M14. Also not know for breaking parts.
H
Most of these pros & cons are actually the other way round. :) Other than that, good that you are doing research.
Correct, or completely irrelevant. Which should answer the original question quite nicely. It would appear the research is based on forum opinion and not experience or evidence.
OP, here is a thread on how "dirty" a short stroke piston rifle gets.......it isn't a huge round count as I generally keep my rifles clean.
https://www.m4carbine.net/showthread.php?142245-HK-MR762-Range-Day
M14. Also not know for breaking parts.
H
No one shoots them enough to know. The M1 and M1 Carbine both break parts.
It's good that you're doing research.
Your table should probably include the in-line short stroke piston system used in the standard AR design.
There are several other factors that should be considered including:
handling, balance, serviceability, simplicity, durability and cost.
The standard AR is very efficient and reliable when made from quality parts.
If you're looking to choose your first rifle, look no further than a quality DI AR.
I've been doing some research into long versus short stroke piston designs for the AR pattern. It seems of my collection of pros/cons, the long stroke piston design is the better design, yet the long stroke is not the choice among the top major manufacturers (HK, Sig, Colt, LWRC, FN SCAR, others).
The short stroke popularity among the top manufacturers has me asking what am I missing from my pro/con list?
...
Can someone please enlighten me or add to my list?
Thanks,
Jake
I've been doing some research into long versus short stroke piston designs for the AR pattern. It seems of my collection of pros/cons, the long stroke piston design is the better design, yet the long stroke is not the choice among the top major manufacturers (HK, Sig, Colt, LWRC, FN SCAR, others).
The short stroke popularity among the top manufacturers has me asking what am I missing from my pro/con list?
One advantage I have as a civilian is that I get to decide what is best for me and my situation. Perhaps it is just a matter of logistics; the short stroke manufacturers were given logistical military requirements that differ from my home logistics?
Can someone please enlighten me or add to my list?
21703
Thanks,
Jake
Spare parts. Proprietary-design piston gun goes inert, who you gonna call? DI gun burps, reach for your parts box.
GSM
Failure2Stop
11-29-13, 20:23
In these days of operating system enthrallment people tend to focus on specific firearms and the operating system instead of the actual result.
Further, the concept of "long stroke" and "short stroke" piston systems can be broken down into vastly more specific terms/definitions to more accurately describe what the system is doing.
Operating system alone really means very little as to how the system will function in real use. I can point out numerous examples of both types of piston operation that are poor performers. The counter to that coin is that there is really only one "inline piston" operating system: the Stoner designed AR15 family. The faults and successes of this system is linked to basically only one example.
So, in short, one cannot claim anything about any operating system without talking about specific firearms, and their operation generally has less to do with the "what" than with the "how".
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Suwannee Tim
11-30-13, 06:38
My first AR was a piston, an LMT, I wanted to learn for myself what the great attraction of piston guns was. Answer, none. Piston guns have no meaningful advantages over DI for most users. DI is cheaper, lighter, easier supported. That said, I can't argue against you getting a piston if that is what you want because that's exactly what I did and I don't regret it.
Where in the original post does the OP even mention DI? I might have missed it, but where did he ask or should I get a DI rifle? Heck he didn't even say he was buying a rifle as I remember, he was simply asking a question about two piston systems.
Why does EVERY single piston thread result in a gang of DI thugs rolling in to defend the holy DI chalice. I own both and both send lead down range, anyone who thinks otherwise should be sending more lead down range,
It isn't even on topic.
sinlessorrow
11-30-13, 15:33
In these days of operating system enthrallment people tend to focus on specific firearms and the operating system instead of the actual result.
Further, the concept of "long stroke" and "short stroke" piston systems can be broken down into vastly more specific terms/definitions to more accurately describe what the system is doing.
Operating system alone really means very little as to how the system will function in real use. I can point out numerous examples of both types of piston operation that are poor performers. The counter to that coin is that there is really only one "inline piston" operating system: the Stoner designed AR15 family. The faults and successes of this system is linked to basically only one example.
So, in short, one cannot claim anything about any operating system without talking about specific firearms, and their operation generally has less to do with the "what" than with the "how".
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There are a few that user the inline system. I know the Khaybar 2002 does, the Daewoo K11's 5.56 system uses it, and there's a few others, but no denying the most common is of course the AR platform with numerous makers offering advancements such as the SR-15.
Where in the original post does the OP even mention DI? I might have missed it, but where did he ask or should I get a DI rifle? Heck he didn't even say he was buying a rifle as I remember, he was simply asking a question about two piston systems.
Why does EVERY single piston thread result in a gang of DI thugs rolling in to defend the holy DI chalice. I own both and both send lead down range, anyone who thinks otherwise should be sending more lead down range,
It isn't even on topic.
Agreed. A little ridiculous.
Where in the original post does the OP even mention DI? I might have missed it, but where did he ask or should I get a DI rifle? Heck he didn't even say he was buying a rifle as I remember, he was simply asking a question about two piston systems.
Why does EVERY single piston thread result in a gang of DI thugs rolling in to defend the holy DI chalice. I own both and both send lead down range, anyone who thinks otherwise should be sending more lead down range,
It isn't even on topic.
Well, the OP does say, in his first sentance, 'I've been doing some research into long versus short stroke piston designs for the AR pattern.' He doesn't ask for a comparison to DI (or seem to realize that the standard AR uses a short stroke piston made up of the bolt and bolt carrier, but that's another jihad) but in that context a comparison to DI seems warranted.
H
The legacy system does NOT use a short stroke piston.
I agree that the OP does not contain any reference to the DI system at all. It does seem that the OP has the pros and cons mixed up though, but I also agree with F2S that you have to review the design on a gun to gun basis.
That said, is there an AR type gun that uses a long strike piston?
The legacy system does NOT use a short stroke piston.
I agree that the OP does not contain any reference to the DI system at all. It does seem that the OP has the pros and cons mixed up though, but I also agree with F2S that you have to review the design on a gun to gun basis.
That said, is there an AR type gun that uses a long strike piston?
PWS?
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Thanks, wasn't aware of that.
BRB... Getting my popcorn ready for this one.
Ha! Someone knew what was going to happen here. I must have read right past this post originally.
Looks like it was a drive by anyway.
Thanks for the comments. I came to these questions when researching for an AR piston and selecting a manufacturer to purchase from. The piston designs have a lot of variety, and therefore required further understanding.
My intention is to get a better understanding of the successful short and long stroke implementations on the AR pattern. I'm very interested in the mechanical differences and the implied physics.
Let me share some of reasons why I scored the pros/cons:
Most of these pros & cons are actually the other way round.
a. Forget the "parts are hitting..."
b. longstroke pistons have inevitably more reciprocating mass -> """more""" recoil
c. carrier tilt has nothing to do with the type of piston
d. functioning dirty has nothing to do with the type of piston, but long strokes do tend to get dirtier faster.
e. ability cycle "poor ammo" has nothing to do with the type of piston
f. short strokes stay much cleaner and thus are easier to clean
g. no real difference in weight (as in terms of balance or lugging it around)
h. accuracy is not related to piston type
a. Parts hitting each other sound like stress.
b. The short stroke feels more punchy; shorter and higher pressure to do the same work of a longer piston.
c. In the AR pattern it seems most (maybe all) short stroke have a tilt issue.
d. The mass of the long stoke makes malfunction less likely do to grime.
e. The mass of the long stoke makes malfunction less likely do to poor ammo.
f. The long stroke has less parts and easier to disassemble, making it easier to clean.
g.
h. Short stroke cycles faster, giving an advantage for full auto and quicker recovery.
Hi Jake,
Thanks for stopping back.
I thought you may be rearching in preparation for a purchase.
To receive the best guidance, it may help to clarify your situation and relate if its similar to one of the following.
A) this is your first AR purchase, you've heard questionable things about Bushmaster DI guns, want the "best" and don't want it to "jam" with Tula steel case ammo, but have little practical experience with the AR platform.
B) you have a safe full of top tier ARs, you know they are stone cold reliable if a few simple procedures are followed are a little bored with them and just want something "different" to round out the collection.
The answers you'll receive will be quite a bit different depending on where you're at.
Thanks for the comments. I came to these questions when researching for an AR piston and selecting a manufacturer to purchase from. The piston designs have a lot of variety, and therefore required further understanding.
My intention is to get a better understanding of the successful short and long stroke implementations on the AR pattern. I'm very interested in the mechanical differences and the implied physics.
BufordTJustice
12-02-13, 04:34
Hi Jake,
Thanks for stopping back.
I thought you may be rearching in preparation for a purchase.
To receive the best guidance, it may help to clarify your situation and relate if its similar to one of the following.
A) this is your first AR purchase, you've heard questionable things about Bushmaster DI guns, want the "best" and don't want it to "jam" with Tula steel case ammo, but have little practical experience with the AR platform.
B) you have a safe full of top tier ARs, you know they are stone cold reliable if a few simple procedures are followed are a little bored with them and just want something "different" to round out the collection.
The answers you'll receive will be quite a bit different depending on where you're at.
THIS.
Failure2Stop
12-02-13, 06:26
Thanks for the comments. I came to these questions when researching for an AR piston and selecting a manufacturer to purchase from. The piston designs have a lot of variety, and therefore required further understanding.
My intention is to get a better understanding of the successful short and long stroke implementations on the AR pattern. I'm very interested in the mechanical differences and the implied physics.
Let me share some of reasons why I scored the pros/cons:
a. Parts hitting each other sound like stress.
b. The short stroke feels more punchy; shorter and higher pressure to do the same work of a longer piston.
c. In the AR pattern it seems most (maybe all) short stroke have a tilt issue.
d. The mass of the long stoke makes malfunction less likely do to grime.
e. The mass of the long stoke makes malfunction less likely do to poor ammo.
f. The long stroke has less parts and easier to disassemble, making it easier to clean.
g.
h. Short stroke cycles faster, giving an advantage for full auto and quicker recovery.
False.
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Grand58742
12-02-13, 07:44
The legacy system does NOT use a short stroke piston.
I agree that the OP does not contain any reference to the DI system at all. It does seem that the OP has the pros and cons mixed up though, but I also agree with F2S that you have to review the design on a gun to gun basis.
That said, is there an AR type gun that uses a long strike piston?
Adcor Bear kind of. It's what they call a hybrid design. Has the oprod attached to the BCG and goes out to the gas block, but the actual piston itself is still separate up in the gas block itself.
If the piston moves less than the length of one cartridge it's a short stroke system. Longer than one cartridge length makes it a long stroke. Op rod (or absence doesn't enter into it).
SCAR: the piston moves less than one cartridge length and drives the bolt carrier directly, making it a short stroke design.
AK: the piston is attached to the bolt carrier by a rivet and cycles more than one cartridge length during operation, making it a long stroke design.
H
So, how far does the piston in the DI AR move?
And what does that make it?
So, how far does the piston in the DI AR move?
And what does that make it?
Fraction of an inch if you're considering the Bolt a piston inside of the Carrier.
Failure2Stop
12-02-13, 11:57
So, how far does the piston in the DI AR move?
And what does that make it?
Like I said above, linking performance to non-descriptive/non-pertinent/non-standard name of the operating system is like linking vehicle performance to the name of the engine instead of what it actually delivers.
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Crow Hunter
12-02-13, 12:21
Thanks for the comments. I came to these questions when researching for an AR piston and selecting a manufacturer to purchase from. The piston designs have a lot of variety, and therefore required further understanding.
My intention is to get a better understanding of the successful short and long stroke implementations on the AR pattern. I'm very interested in the mechanical differences and the implied physics.
Let me share some of reasons why I scored the pros/cons:
a. Parts hitting each other sound like stress. If the gun is designed correctly, it doesn't matter. Just like a hammer hitting an anvil, the hammer doesn't break the anvil
b. The short stroke feels more punchy; shorter and higher pressure to do the same work of a longer piston.It is all dependent on the recoiling mass. The cartridge is going to put out the same amount of recoil energy, how it translates to your shoulder is related to how quickly the mass of the system is accelerated and decelerated and how.
c. In the AR pattern it seems most (maybe all) short stroke have a tilt issue. Not a function of the type of gas system. It is a function of the eccentric loading on the bolt carrier group that wasn't designed originally for eccentric (off center) loading. I guess a long stroke piston could "stop" tilt by having the pistol/op rod trapped and prevent the carrier from tilting, but then the wear will just be somewhere else other than the back of the carrier. Unless steps were taken to avoid this
d. The mass of the long stoke makes malfunction less likely do to grime.Maybe, it also means there will be more mass to accelerate when it is cold and other times there is reduced gas energy, such as low powered ammo. The higher reciprocating mass can also play havoc on optics not designed around it. Re SCAR.
e. The mass of the long stoke makes malfunction less likely do to poor ammo. See above, it could make it worse as there is more mass there is more energy required to get it to start moving.
f. The long stroke has less parts and easier to disassemble, making it easier to clean. I would think it would depend on the design. But even worst case you might have 2 or 3 extra parts (piston and return spring and op rod if it isn't connected to the carrier)
g.
h. Short stroke cycles faster, giving an advantage for full auto and quicker recovery. On 2 identical rifles with identical weights, I think this would be so. But in reality there will usually be other differences that will influence this.
Buy Ian Hoggs Military Small Arms of the 20th Century. He has a very good description, in laymen's terms of different operating systems. He also has quite a bit of details about the weapons themselves.
As to your questions:
See mine underlined.
ETA: Edited so you could read it.:rolleyes:
ETA 2: Also, reading through it, you will see Mr. Leuba's point about what rifles did and didn't work, regardless of their operating systems. There are quite a few failures in there. :)
I agree.
We all know DI works fine, regardless of what anyone tries to call it.
It was really just a rhetorical question in response to the assertion that DI is NOT short stroke.
Like I said above, linking performance to non-descriptive/non-pertinent/non-standard name of the operating system is like linking vehicle performance to the name of the engine instead of what it actually delivers.
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a. Parts hitting each other sound like stress.
b. The short stroke feels more punchy; shorter and higher pressure to do the same work of a longer piston.
c. In the AR pattern it seems most (maybe all) short stroke have a tilt issue.
d. The mass of the long stoke makes malfunction less likely do to grime.
e. The mass of the long stoke makes malfunction less likely do to poor ammo.
f. The long stroke has less parts and easier to disassemble, making it easier to clean.
g.
h. Short stroke cycles faster, giving an advantage for full auto and quicker recovery.
This post is like a 2006 ARF.com time warp.
There are a bunch of companies making good piston rifles with solid track records. Most, if not all, are short stroke. Worry less about the details of the operating system and more about getting a reliable rifle that won't force you to second mortgage your home. (IE: In your budget.)
ETA: I'm not trying to offend you OP, it just reminded me of some very interesting threads from that time period.
So, how far does the piston in the DI AR move?
And what does that make it?
Short stroke as the piston moves less than one cartridge length in relation to its cylinder based on the reasoning in this Armalite tech note:http://www.armalite.com/images/Tech%20Notes%5CTech%20Note%2054,%20Gas%20vs%20Op%20Rod%20Drive,%20020815.pdf
The gas expands within a donut shaped gas cylinder within the carrier. Because the bolt is prevented from moving forward by the barrel, the carrier is driven to the rear by the
expanding gasses and thus converts the energy of the gas to movement of the rifle’s parts.
The bolt bears a piston head and the cavity in the bolt carrier is the piston sleeve.
The other example of a 'DI' rifle that's commonly used, the AG-42, is also using a gas piston on the bolt carrier and a cylinder formed from the receiver. Based on the definition above, it's be a long stroke gas system.
21809
H
I agree.
We all know DI works fine, regardless of what anyone tries to call it.
It was really just a rhetorical question in response to the assertion that DI is NOT short stroke.
Granted, going off the view that the bolt is a piston in a DI gun, maybe short stroke is the correct term for that part of the system.
However, the part most commonly associated with a short or long stroke piston system is the oprod or actuating rod.
That was the basis for my assertion. I assume you see my point?
Granted, going off the view that the bolt is a piston in a DI gun, maybe short stroke is the correct term for that part of the system.
However, the part most commonly associated with a short or long stroke piston system is the oprod or actuating rod.
That was the basis for my assertion. I assume you see my point?
Not all gas operated rifles have a op rod.
The AK would be a long stroke rifle that doesn't have a separate op rod while the SCAR is a short stroke design that doesn't have a separate op rod either.
H
Are we talking about the AR piston guns or not?
Granted, going off the view that the bolt is a piston in a DI gun, maybe short stroke is the correct term for that part of the system.
However, the part most commonly associated with a short or long stroke piston system is the oprod or actuating rod.
That was the basis for my assertion. I assume you see my point?
This is a re-occurring argument in which there is no foundation. You don't have to believe me in saying that Ar15 is a DI system whether or not there is piston somewhere in the system or not. Mr. Vickers put the argument into rest pretty nicely few moons ago (and discredited the mentioned Armalite doc in the process too).
I'd like to stick to the discussion of AR pistons that do not include DI; in the context when I hear manufacturers designing short/long stroke pistons. I presume each design has tradeoffs, and I've been trying to learn more about what they are, and what real difference it makes. There's many popular short stroke pistons, but very few long stroke pistons (e.g. PWS). And this had me wondering if the long stroke is disadvantaged over the short, or if the long stroke would offer advantages to the civilian that aren't shared with the military (the military may have different requirements than I do as a civilian).
PatrioticDisorder
12-03-13, 13:49
I'd like to stick to the discussion of AR pistons that do not include DI; in the context when I hear manufacturers designing short/long stroke pistons. I presume each design has tradeoffs, and I've been trying to learn more about what they are, and what real difference it makes. There's many popular short stroke pistons, but very few long stroke pistons (e.g. PWS). And this had me wondering if the long stroke is disadvantaged over the short, or if the long stroke would offer advantages to the civilian that aren't shared with the military (the military may have different requirements than I do as a civilian).
You find much love for piston ARs here (and for good reason), they really don't offer much to most people unless scolding hot forends, proprietary parts, higher likelihood of a broken part, front heavy and over priced are your thing. Truthfully the only piston that has any appeal to me at all (and still over priced) is the LWRC six8 UCIW and that is only because it's built as a 6.8spc II from the ground up. If DD, BCM, Colt or LMT make a rifle built around the 6.8 pmags I'd completely lose interest In the LWRC.
This is a re-occurring argument in which there is no foundation. You don't have to believe me in saying that Ar15 is a DI system whether or not there is piston somewhere in the system or not. Mr. Vickers put the argument into rest pretty nicely few moons ago (and discredited the mentioned Armalite doc in the process too).
With all due respect, the original AR design does use a piston and it is part of the bolt and it does use rings to seal the cylinder which is located inside the carrier. Eugene Stoner states in his original patent that his design is NOT a direct impingement as it uses an expansion chamber and the US patent office accepted Stoner's claim.
If you study true DI systems, such as used on the Ljungman, you'll see that the gas impinges directly to an external surface of the carrier and does not use a piston or expansion chamber
With all due respect, the original AR design does use a piston and it is part of the bolt and it does use rings to seal the cylinder which is located inside the carrier. Eugene Stoner states in his original patent that his design is NOT a direct impingement as it uses an expansion chamber and the US patent office accepted Stoner's claim.
If you study true DI systems, such as used on the Ljungman, you'll see that the gas impinges directly to an external surface of the carrier and does not use a piston or expansion chamber
With all due respect to you, the point you are constantly trying to get across is irrelevant.
Regardless of whether or not the bolt acts as a piston in the carrier, everyone understands what is meant by saying "piston AR"; HK416/MR223, LWRC, Colt 6940P, Adams Arms, LMT piston guns and so forth, ad nauseum. Ie, no gas is siphoned back to the receiver/bolt carrier, via a gas tube.
Trying to distract from the topic at hand by using that argument is really pointless.
To the OP:
My experience with piston AR's comes from being issued and using the HK416 for 5 years. The HK416 is labeled a short stroke piston rifle, and the mechanism consists of a gas piston, a piston rod and a piston BCG. The piston itself is housed in the gas block and is fitted with three piston rings to form a seal. The piston rod feeds into the gas piston in one end, and is fed in to the upper through a bushing in the upper receiver at the rear end.
When fired, gas is siphoned off through the gas port in the barrel and into the gas block. Some of this gas is used to push the piston to the rear, which in turn pushes the piston rod to the rear. There is only a few milimeters separating the piston rod and the strike face of the bolt carrier guide lug (where the carrier key sits in a normal AR), and as soon as the piston rod makes contact with the strike face on the carrier guide, the bolt carrier starts moving to the rear. The rest is the same as with a normal DI gun.
Personally I feel that many of the perceived negatives are exaggerated, and don't see any issue with excessive recoil, hot handguards/forends, weight issues or reduced accuracy. That is not to say that many of the very experienced shooters on here cannot see a tangible difference in results down range when using a DI gun compared to a piston gun, but for most it is nothing more than irrelevant/negligible minutiae.
I think it handles fine:
https://www.youtube.com/watch?v=WGP1t-qn6po
The same in FA (10 shot bursts at 20, 10 and 5 meters, ):
https://www.youtube.com/watch?v=x1t1ZSpot-4
Maintenance is very easy, no real build up of carbon in the upper or lower, and lube stays in place pretty well. I replaced the piston once, as the gas rings were worn, other than that the gun needed no parts replacements what so ever. No signs of carrier tilt.
I think that the main reason behind the cautionary stance many on here have with regards to non-DI AR's, is that the quality of some designs have been found less than ideal, so that any perceptible gain is questionable at best. Especially retro fit kits. If you want a piston AR, it is good sense to go with a well known manufacturer that usually puts out good products, and avoid any sensationalistic selling points.
I also believe that many of the detractors of piston ARs on the internet have little to no experience with one, and are just parroting what they have heard or read.
As to your list in the OP, I do not agree with the cons you attribute to a short stroke piston AR. They do not mirror my own experiences, although I was spoiled by being issued the piston gun that is regarded as the best. Other designs might not work that well.
Again, if you decide to go the piston route, stick with a known quantity name and avoid retro fit kits and sensational selling points. If not, a DI gun will be just as good, with some added benefits related to parts availability and selection. That is probably the biggest detractor for the piston AR's in my mind, especially when it comes to handguards, that there really aren't that many options that are available....
Excellent all around advice from an actual HK416 user.
OP:
I think the main reason there are few long stroke AR designs is that there is not much physical room at the front of the receiver, above the barrel extension for a large diameter piston head to pass.
A short stroke design can have all the large diameter stuff above the barrel, in front of the receiver, with a relatively narrow op rod passing through the front of the receiver.
To the OP:
My experience with piston AR's comes from being issued and using the HK416 for 5 years. The HK416 is labeled a short stroke piston rifle, and the mechanism consists of a gas piston, a piston rod and a piston BCG. The piston itself is housed in the gas block and is fitted with three piston rings to form a seal. The piston rod feeds into the gas piston in one end, and is fed in to the upper through a bushing in the upper receiver at the rear end.
Crow Hunter
12-03-13, 17:36
I'd like to stick to the discussion of AR pistons that do not include DI; in the context when I hear manufacturers designing short/long stroke pistons. I presume each design has tradeoffs, and I've been trying to learn more about what they are, and what real difference it makes. There's many popular short stroke pistons, but very few long stroke pistons (e.g. PWS). And this had me wondering if the long stroke is disadvantaged over the short, or if the long stroke would offer advantages to the civilian that aren't shared with the military (the military may have different requirements than I do as a civilian).
There are more rifles designed with short stroke pistons than long stroke overall.
Based on my reading short stroke is the most common. Of course, not all those designs were successful...
That being said, the most wide spread and used rifle in the history of mankind is a long stroke piston(AK-47 and derivatives).
One disadvantage of long stroke over short stroke is heat. Both pistons are exposed to combustion gasses. The short stroke has a "thermal break" that keeps that heat contained within the piston/expansion chamber since it is only in direct contact with the operating rod during operation. The rest of the time it will hav
e some air between the 2 components that give some insulating effect. So the tolerancing requirements for heat expansion/contraction can be isolated to just that area.
On the long stroke design, the entire assembly is exposed to the combustion gasses through conduction and a design has to be toleranced to allow for expansion and contraction along the whole length.
Personally, if I were really wanting the advantages of a piston, I would go with a rifle designed from the ground up for a piston. But failing that, I would go short stroke over long stroke.
Arctic, how long do piston rings in the 416 last? In a DI AR I assume 3-5k rounds between services.
I'd say between 8-10k on average, with variations due to harsher/milder firing schedules and so forth.
My gun went 10k plus before they changed it out.
During the inspection, the armorer said it was a bit loose. It didn't drop out of the gas block when the muzzle was pointed up, but there wasn't too much friction left either.
BufordTJustice
12-03-13, 18:30
With all due respect, the original AR design does use a piston and it is part of the bolt and it does use rings to seal the cylinder which is located inside the carrier. Eugene Stoner states in his original patent that his design is NOT a direct impingement as it uses an expansion chamber and the US patent office accepted Stoner's claim.
If you study true DI systems, such as used on the Ljungman, you'll see that the gas impinges directly to an external surface of the carrier and does not use a piston or expansion chamber
Mist, have you been reading the ACTUAL PATENT again? ;)
https://www.google.com/patents/US2951424?dq=eugene+stoner&hl=en&sa=X&ei=0HSeUqLSH8ru2AXfwoFA&sqi=2&pjf=1&ved=0CEQQ6AEwAg (https://www.google.com/patents/US2951424?dq=eugene+stoner&hl=en&sa=X&ei=0HSeUqLSH8ru2AXfwoFA&sqi=2&pjf=1&ved=0CEQQ6AEwAg)
From Stoner's patent, courtesy of the USPO and Google Patent search:
"This invention pertains to automatic rifle mechanisms and in particular to the gas system employed for operating the bolt and bolt carrier.
The most widely used method of operation of automatic rifles today is the conventional gas cylinder, piston and actuating rod assembly; the only other system now in production use being the recoil actuating system. The blow-back or inertia block system is usually reserved for weapons using lower powered ammunition, such as pistol and .22 caliber weapons.
It is a principal object of this invention to utilize the basic parts of an automatic rifle mechanism such as the bolt and bolt carrier to perform a double function. This double function consists of the bolts primary function to lock the breach against the pressure of firing, and secondarily, to act as a stationary piston to actuate the automatic rifle mechanism. The primary function of the bolt carrier is to lock and unlock the bolt by rotating it and to carry it back and forth in the receiver. The secondary function of the bolt carrier is to act as a movable cylinder to actuate the automatic rifle mechanism. By having the bolt carrier act as a movable cylinder and the bolt act as a stationary piston, the need for a conventional gas cylinder, piston and actuating rod assembly is eliminated.
It is an object of this invention to provide a gas system which is lighter and less expensive to produce because of its simplicity than the present gas systems now used in automatic rifle mechanisms.
It is another object of this invention to utilize the energy of the expanding gas developed by the firing of the weapon, for actuating the automatic rifle mechanism directly by use of a metered amount of the gas coming from the barrel. This invention is a true expanding gas system instead of the conventional impinging gas system. By utilization of a metered amount of gas from the barrel, the automatic rifle mechanism is less sensitive to different firing pressures caused by variations in the propelling charge. It is therefore still another object of this invention, to provide a rifle mechanism which is not affected by variations in the propelling charge.
A further object of this invention is to provide smoother operation and longer life of the working parts of the automatic rifle mechanism. Since, in this invention, the actuating force is transmitted directly down the center line of the barrel and the bolt mechanism to the shoulder of the shooter, all of the of]? center loads found in most other types of gas actuated Weapons are eliminated. It will therefore be obvious because of this factor that another object of this invention is to cut down on climb which occurs during automatic firing operations."
It seems that, if one cares to read the ACTUAL PATENT, as MistWolf did, that there is an important distinction to be drawn here. A distinction so important, that it highlights the redundancy of an external piston system, such as the ones being discussed in this thread, as items that Stoner went to great length to collapse into his double-function for various reasons stated above.
Face the facts, folks. The AR15 always was a piston gun. I'll cite the last paragraph quoted above as to WHY it is the best setup.
With all due respect to you, the point you are constantly trying to get across is irrelevant...
I find it quite relevant. Many shooters have the misconception the AR is an inferior design because it is a direct impingement system, lacking a piston. This leads them to believe that adding a piston will solve all the "problems" an AR has. Continuing the belief an AR is a DI system is the distraction. How can we discuss the merits of each system when we do not acknowledge what each system really is?
How can we have a meaningful dialogue without identifying the facts? For example, critics of the piston & tappet design (like the HK416) claim it is heavier than one with the Stoner system. But where does the extra weight come from? If all else is equal, it's the tappet. Yet, how much does the tappet weigh? Two ounces? Four? Or does the upper weigh more because a heavier profile barrel is used? I've never weighed a tappet from an AR P&T upper, but it looks smaller than the FAL tappet and the FAL tappet doesn't seem to weigh very much. The argument is flung about that a P&T upper is heavier simply because it's a P&T upper without any empirical data to back it up- I admit to being guilty of making the same claim myself.
I am challenging the conventional wisdom but it's because I've been looking into this for myself, to either verify what's right or debunk the myths. If I simply accepted the conventional wisdom that the AR is a DI system, I would have missed out on all the other things I've discovered looking for the truth of it
I find it quite relevant. Many shooters have the misconception the AR is an inferior design because it is a direct impingement system, lacking a piston. This leads them to believe that adding a piston will solve all the "problems" an AR has. Continuing the belief an AR is a DI system is the distraction. How can we discuss the merits of each system when we do not acknowledge what each system really is?
How can we have a meaningful dialogue without identifying the facts? For example, critics of the piston & tappet design (like the HK416) claim it is heavier than one with the Stoner system. But where does the extra weight come from? If all else is equal, it's the tappet. Yet, how much does the tappet weigh? Two ounces? Four? Or does the upper weigh more because a heavier profile barrel is used? I've never weighed a tappet from an AR P&T upper, but it looks smaller than the FAL tappet and the FAL tappet doesn't seem to weigh very much. The argument is flung about that a P&T upper is heavier simply because it's a P&T upper without any empirical data to back it up- I admit to being guilty of making the same claim myself.
I am challenging the conventional wisdom but it's because I've been looking into this for myself, to either verify what's right or debunk the myths. If I simply accepted the conventional wisdom that the AR is a DI system, I would have missed out on all the other things I've discovered looking for the truth of it
Completely irrelevant!
Who in this thread said anything about DI AR's being bad? DI AR's were not even mentioned until the chorus of just buy a DI AR was inserted!
We're all impressed with the patent information that I am sure no one has ever seen before, in numerous other threads, on numerous forums, since 2005, when HK improved the AR reliability, service life and maintainability of the platform by releasing the HK416 which has since been copied by numerous high end AR makers.
I can play this game too. See how it works? It is very nearly 2014 and the song remains the same, in spite of a very much combat and private ownership proven track record.
Rodger? Over and out! :)
Heh!
Just to be clear, there are still lot's of shooters who are "just getting into ARs" who want to know what the "best piston upper is" because it's better than DI. If more folks realized how the AR really works, it would be easier to explain to them how to choose an AR for themselves.
Also, I have nothing against the HK416. Arctic1 has long since convinced me it's a rifle worthy to ride the trail with
Heh!
Just to be clear, there are still lot's of shooters who are "just getting into ARs" who want to know what the "best piston upper is" because it's better than DI. If more folks realized how the AR really works, it would be easier to explain to them how to choose an AR for themselves.
Also, I have nothing against the HK416. Arctic1 has long since convinced me it's a rifle worthy to ride the trail with
That is completely fair mist.......but wouldn't it make more sense to just start that thread rather than posting it in this one? I'm not accusing you of hating......:)
This is the type of information I am looking for...
I think the main reason there are few long stroke AR designs is that there is not much physical room at the front of the receiver, above the barrel extension for a large diameter piston head to pass.
A short stroke design can have all the large diameter stuff above the barrel, in front of the receiver, with a relatively narrow op rod passing through the front of the receiver.
One disadvantage of long stroke over short stroke is heat. Both pistons are exposed to combustion gasses. The short stroke has a "thermal break" that keeps that heat contained within the piston/expansion chamber since it is only in direct contact with the operating rod during operation. The rest of the time it will have some air between the 2 components that give some insulating effect. So the tolerancing requirements for heat expansion/contraction can be isolated to just that area.
On the long stroke design, the entire assembly is exposed to the combustion gasses through conduction and a design has to be toleranced to allow for expansion and contraction along the whole length.
Personally, if I were really wanting the advantages of a piston, I would go with a rifle designed from the ground up for a piston. But failing that, I would go short stroke over long stroke.
Piston guns are getting better. Early LWRCs broke the key off of the carriers and mushroomed parts in the piston system. Those issues are resolved. The HK had significant teething issues also, and by the A5 model they too appear to be resolved. To me the advantages of piston guns are obvious in two places: they burn lube off more slowly and they require next to no dwell time since the gas needs to move only to the face of the piston. In return you have more recoil and need to have a removable handguard to service the gas system--the most brilliant example being the 416. In 10.5" and shorter ARs I see a benefit to a piston gun. My SBR is an 11.5" DI gun. Your average AR buyer wants a 16" gun and for that there is absolutely no need for a piston.
If I bought a piston upper it would be a PWS. I like long stroke pistons.
Piston guns are getting better. Early LWRCs broke the key off of the carriers and mushroomed parts in the piston system. Those issues are resolved. The HK had significant teething issues also, and by the A5 model they too appear to be resolved. To me the advantages of piston guns are obvious in two places: they burn lube off more slowly and they require next to no dwell time since the gas needs to move only to the face of the piston. In return you have more recoil and need to have a removable handguard to service the gas system--the most brilliant example being the 416. In 10.5" and shorter ARs I see a benefit to a piston gun. My SBR is an 11.5" DI gun. Your average AR buyer wants a 16" gun and for that there is absolutely no need for a piston.
If I bought a piston upper it would be a PWS. I like long stroke pistons.
That is completely fair mist.......but wouldn't it make more sense to just start that thread rather than posting it in this one?
The impact and relevance would have been lost
I'm not accusing you of hating......:)
Of course not! :ph34r:
Piston guns are getting better. Early LWRCs broke the key off of the carriers and mushroomed parts in the piston system. Those issues are resolved. The HK had significant teething issues also, and by the A5 model they too appear to be resolved. To me the advantages of piston guns are obvious in two places: they burn lube off more slowly and they require next to no dwell time...
Actually, dwell time has nothing to do with it. The bullet has exited the muzzle before the gas can get the piston moving. Also, the time it takes the bullet to travel and extra inch or two is very, very short. For example, tests show that the bullet of an M14 is several feet downrange before the piston starts moving. However, distance from gas port to muzzle is still important as it increases the time it take the bore to drop to atmospheric pressure at the point of the gas port. If the pressure drops too soon, the gas won't have enough time to get things moving. We see this when cutting an FAL barrel down to 16". Either the port has to be opened wide so the cylinder will charge fast enough, or the gas port must be moved closer to the chamber.
Even so, if the piston were to begin moving before the bullet exited the muzzle, the extra distance would make no difference at all because the piston would begin moving with the bullet at the same distance from the port regardless of how much extra distance there was between port & muzzle
DreadPirateMoyer
12-04-13, 01:20
Are people in this thread still arguing over the semantics of what piston vs. DI vs. long stroke vs. short stroke vs. whatever-the-hell-else means? Yeesh. Get over yourselves already. It was completely obvious what the OP asked, and all of you are doing nothing but derailing his thread because of some weird semantic argument regarding DI rifles that the OP didn't even bring up.
Whateverthehellyouwanttocallit, the question is about the differences between long-stroke and short-stroke piston/op-rod/whatever systems and their comparative pros-cons. It had nothing to do with DI rifles. It had nothing to do with piston vs. DI rifles. It had nothing to do with what we should actually be calling these systems to be technically correct. It was about long-stroke and short-stroke pros and cons, and every single one of you posting and arguing in this thread knew exactly what the OP meant and was asking about, regardless of the verbage used. Get on topic already and get over your damned egos.
This place is getting as bad as TOS with the level of trolling, bickering, and inability to stay on topic these days.
ETA: And OP, I have to agree with F2S's advice way-the-hell back on page 1. It's hard to comment on these systems as a generality without discussing specific firearms since many of them are even more nuanced beyond long and short-stroke. What may be true for one short-stroke is not necessarily true for another. Same for long-stroke.
MistWolf, all things I know. In a piston gun the gas must travel to the gas block. In an expanding gas system it must travel back to the carrier. The longer the barrel stays pressurized the easier the latter is.
Has anyone shot both a long and short stroke piston AR? Is there any perceptible difference between the two?
Whateverthehellyouwanttocallit, the question is about the differences between long-stroke and short-stroke piston/op-rod/whatever systems and their comparative pros-cons.
This kinda got lost from view somewhere in the middle....big-time...with a only couple of notable exceptions. It mostly reads like story-topping.
If we could go ahead and drag it all the way back to the original question and keep it there....
http://imageshack.us/a/img18/145/pp2o.jpg
Grand58742
12-04-13, 10:05
Has anyone shot both a long and short stroke piston AR? Is there any perceptible difference between the two?
I've own an Adcor BEAR which again is a hybrid design, but more or less a long stroke and have shot the Sig 516 which is a short stroke. Both had H buffers installed and were the 16 inch carbine versions.
No difference for the most part that I could tell. Sig was a bit heavier up front, but shooting wise, both were about the same in the recoil department.
Has anyone shot both a long and short stroke piston AR? Is there any perceptible difference between the two?
Comparing a PWS and an LWRC the latter has a fast, harsh recoil that threw the dot off the target. It quickly fell back into place but was awkward. The PWS felt much more like a DI gun.
BufordTJustice
12-04-13, 11:34
I've been doing some research into long versus short stroke piston designs for the AR pattern. It seems of my collection of pros/cons, the long stroke piston design is the better design, yet the long stroke is not the choice among the top major manufacturers (HK, Sig, Colt, LWRC, FN SCAR, others).
The short stroke popularity among the top manufacturers has me asking what am I missing from my pro/con list?
One advantage I have as a civilian is that I get to decide what is best for me and my situation. Perhaps it is just a matter of logistics; the short stroke manufacturers were given logistical military requirements that differ from my home logistics?
Can someone please enlighten me or add to my list?
21703
Thanks,
Jake
I have shot every major design of piston AR. From Adams arms derivatives (SIG, M&P, etc.), to Patriot Ordnance Factory, LWRCi, HK, PWS, LMT, Bushmaster (POF derivative), CMMG, etc.
First, it would behoove us to properly define what "short stroke" and "long stroke" is. K.L. Davis did this for us:
"Short Stroke: The piston travels less than the length of the cartridge.
Long Stroke: The piston travels more than the length of the cartridge.
Rule of Thumb: If the piston is attached to the operating rod or mechanism, it is a long stroke; if the piston is separate, it is a short stroke. "
Original post: https://www.m4carbine.net/showthread.php?30430-Short-Stroke-Piston-VS-Long-Stroke-Piston&p=361615#post361615
The only piston gun I have shot that had ANYWHERE near as smooth of a recoil pulse was the PWS system. It lacked the sharp smack of a recoil shot (much like an SKS) that every other short-stroke piston design I've shot possessed. The HK416's I shot were smoother than most, but were ridiculously heavy for that smoothness (one was a 10.5" gun with a light and Aimpoint Micro the other a 16" with same setup and a BC1.0; they were shot on the same day, though were assigned to different LEO's, and both were ridiculously heavy). And, seeing that weight mitigates recoil on a 1:1 basis, the additional weight alone could have been the reason that the HK was smoother than all but the PWS (which was considerably lighter; I didn't shoot them on the same day).
I shot each gun back-to-back at my agency range with my BCM 14.5" middy with MOE and std FS (also, a TLR1-s, pressure switch, MOE foregrip, IWC sling mount, STR, Vltor A5 w/ H4 buffer, FSC556, and T1 in an LT660) and both the 10.5" and 16" guns were considerably heavier than my BCM-upper'd patrol carbine with the listed attachments.
However, neither shot as smoothly or offered the quickness of follow-up shots as my BCM14.5" middy when standing from 50 yards. The piston guns just moved around more...especially when in improvised shooting positions. You can't say it was the muzzle devices because one of the HK's had a BC1.0 and the PWS had an identical muzzle device to my BCM (a PWS FSC556).
Were they finely built guns? Yes.
Were they 100% functioning? Yes.
Did the cheaper one cost almost twice as much as my BCM 14.5" middy? Yes.
Would I rather have either the HK or the PWS over a Bushmaster/DPMS/CMMG/Olympic Arms? Hell yes.
But, what happened after both of those range trips was the guys who owned THOSE GUNS were asking me where to get a gun LIKE MINE. The buddy who was issued the HK 16" quickly became the proud new owner of a BCM14.5" middy w/ perm'd BC1.5. He bought a Vltor A5 conversion kit shortly afterward and he shoots that gun more than his HK when he's not training.
Purely in terms of the shooting experience (when considering that a quality DI AR can be as near to 100% reliability as any machine), why go piston?
My answer to that question: piston AR owners are the Porsche 911 drivers of the gun world. Porsche took a design (engine centerline behind the centerline of the rear axle) that SHOULDN'T work well because of the unconventional design and added complexity, and made it work very well. It seems that piston owners enjoy the added cost, additional R&D, and the emotional "uphill climb" of this struggle.
I don't have the time or money for that. But that's just me.
Hey, you guys trying to turn this into and keep it a piston vs. DI thread (as we've some kind of shortage.....) and whose posts I just deleted:
Once I post in a thread, I'm subscribed to the thread and get notified of your attempts to disregard what I told you to do.
That's how it works. Please bear that in mind, lest I grow wroth.....
(Those posting to attempt to get the others to pay the **** attention....you know who you are, and thanks).
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