The m4, DI, and piston----no this isn't your usual VS thread.

[kal]

recently in the remington acr thread, I was called out for not knowing what I'm talking about. :D

This thread isn't about the acr. It's about two identical rifles with one mechanical difference. I'm just trying to understand something specific here. This goes out to the engineers/ subject experts here.


Let's say there are two M4 rifles. Everything about them is EXACTLY the same, except that the second m4 has a short stroke piston instead of the direct impingment gas tube.

The question is, assuming the ONLY difference between the rifles is that one is piston operated, will the piston operated rifle unlock the bolt faster with much force or slower and easier compared to the DI m4?

[Omega_556]

It depends upon the piston system you are asking about.

ARES
LMT
LWRCi
POF
PWS
and more

[Thomas M-4]

Piston should unlock faster than a DI system. In a DI system the pressure has to build to begin the unlocking process. The piston system will use direct mechanical force to unlock the bolt.
Piston system the gas to operate it comes straight out of the barrel.
DI system the gas has to travel down the gas tube and into the carrier the whole system has more volume compared to a gas piston system .

[kal]

Piston should unlock faster than a DI system. In a DI system the pressure has to build to begin the unlocking process. The piston system will use direct mechanical force to unlock the bolt.

That's exactly what i was thinking.

Now still talking about the same two M4 rifles, one DI the other piston operated, everything else being equal. Knowing that the piston operated rifle would try to unlock the bolt faster than the DI rifle, what features must the piston operated M4 need to SLOW DOWN the initial unlocking of the bolt?

[Thomas M-4]

That's exactly what i was thinking.

Now still talking about the same two M4 rifles, one DI the other piston operated, everything else being equal. Knowing that the piston operated rifle would try to unlock the bolt faster than the DI rifle, what features must the piston operated M4 need to SLOW DOWN the initial unlocking of the bolt?

Good question :confused: Maybe some kind of gas delay built into the gas block?
I dont know if anybody has come up with a gas delayed system it might be worth looking through some online patents to see if you can find something usable.


On a side note didn't the HKG3 use a fluted chamber to help with early extraction that would probable be the easiest solution.
Never mind just realized that probable would not work on a rotating bolt

[Frank Castle]

The pressure is produced virtually instantaneously and truly comparable systems will perform identically. The only significant difference between the two systems is the location in which the gas expands to impart rearward motion of the BCG- in a Piston system it is at the gas block and DI at/near the breech.

[militarymoron]

what features must the piston operated M4 need to SLOW DOWN the initial unlocking of the bolt?

leave the gas rings on to provide some drag on the carrier and make sure that enough of an expansion chamber in the gas block is incorporated into the design.

[militarymoron]

in a Piston system it is at the gas block and DI at/near the breech.

the gas is expanding in a DI system the moment it enters the gas port. the gas tube acts as an expansion chamber as it fills up with gas.

[Frank Castle]

the gas is expanding in a DI system the moment it enters the gas port. the gas tube acts as an expansion chamber as it fills up with gas.was simplifying things to differentiate the two systems- technically the gas is expanding once the primer is struck ;)

[K.L. Davis]

The question is, assuming the ONLY difference between the rifles is that one is piston operated, will the piston operated rifle unlock the bolt faster with much force or slower and easier compared to the DI m4?

There is not even close to enough information here to answer your question... assuming that one is "faster" than the other from this would be just that: an assumption.

[militarymoron]

was simplifying things to differentiate the two systems- technically the gas is expanding once the primer is struck ;)

touche :)

[kennith13]

Won't the piston systems with the adjustable gas vents allow some level of pressure reduction? By venting some of the gas, a reduction in pressure should be affected to slow/weaken the action of the system. I should think that this would be one way to tune the lock time. Now, that may not be directly why they were included in the systems, but the effect they have would allow the same result, even if the vents are not calibrated for this purpose.

Also, would not adjusting the tension of the springs in the system allow tuning of the cycle time for the system itself? As well, would not changing the weight of the reciprocating assemblies?

I should think that a certain amount of this is dialed in by the builders of these systems, or should be if it is not.

The size of the cylinder and piston stroke will have a massive effect as well, on top of the methods of sealing the piston against the cylinder wall.

Everything in the system should be able to be tuned at the factory to minimize the drawbacks. Now, just because it could be done, doesn't mean it is being done. I'm sure each manufacturer has it's own ideas about how best to implement the system.

I'm speaking as generally as possible, here, just exploring the whys as if we have never seen one before. That's the way to figure things out. Look at them as completely foreign objects, and the bits and pieces can make more sense and be more easily analyzed in a logical fashion.

You can't directly control the properties of the gas itself, but you can control it's environment and the systems it interacts with. Everything it interacts with on it's journey can have an effect, if properly controlled.

[Peshawar]

I'm so far from being subject matter competent on this that it's a little absurd. However, in America the ignorant love their opinions... :p

Wouldn't varying the size of the gas port, or using an adjustable gas block allow a manufacturer to tune how fast the piston action works? I would assume, but this may be completely wrong, that the larger the gas port or gas allowed into the system, the faster the action of the piston (within reason).

[RadioActivity]

Would a heavier buffer not delay the unlocking via intertia?

[kennith13]

Would a heavier buffer not delay the unlocking via intertia?

The buffer assembly is going to pretty much be a given in this regard. So will the bolt. The entire reciprocating action will have an effect.

Which brings me to a question. What is the thread author trying to get at? I understand the question, but I don't understand the reason for asking it. He is obviously trying to figure something out, but I should think he probably knows this stuff already.

So, what are we trying to determine, here, and why?

[kal]

Would a heavier buffer not delay the unlocking via intertia?

I'm sure it would. But what about a recoil spring that provides a little bit of constant pressure against the carrier, like in an AK. I'm pretty sure the ar15 rifle don't have the buffer spring providing force against the carrier when it's fully in battery.


Which brings me to a question. What is the thread author trying to get at? I understand the question, but I don't understand the reason for asking it.

I was trying to understand if a piston rifle is capable of cycling the action harder than a DI rifle. In theory it seems to be true so I figured there must be design features that tune the rifle to operate correctly without unneccessary wear and tear on the operating group. Now the question is HOW MANY features does a piston rifle need or can use for reliable function, IF it is true that a piston rifle can and will try to unlock the bolt with much more force than a DI rifle.

So far there is...
-gas port diameter
-gas port location
-volume/space in the gas block
-bolt design
-??????

[kennith13]

I'm sure it would. But what about a recoil spring that provides a little bit of constant pressure against the carrier, like in an AK. I'm pretty sure the ar15 rifle don't have the buffer spring providing force against the carrier when it's fully in battery.



I was trying to understand if a piston rifle is capable of cycling the action harder than a DI rifle. In theory it seems to be true so I figured there must be design features that tune the rifle to operate correctly without unneccessary wear and tear on the operating group. Now the question is HOW MANY features does a piston rifle need or can use for reliable function, IF it is true that a piston piston can and will try to unlock the bolt much more forcefully than a DI rifle.

So far there is...
-gas port diameter
-gas port location
-volume/space in the gas block
-bolt design
-??????

There is also everything I mentioned in my other post.

Piston stroke and bore
Springs directly associated with the piston system
Reciprocating mass of the piston system
Reciprocating mass of the rifle components
Gas vent setting

The smart thing to do would be to tailor all these things just a bit in the design of the system so their collective effect would be to dampen the shock loads on the rifle's action. A decent spring on the operating rod, a cylinder and piston with reasonable displacement, a slightly heavy operating rod, a slightly more heavy buffer, auto bolt carrier... and so on.

Given the whole system to work with, there is plenty that can be done to keep it from cycling too hard. Does everyone do it? I couldn't tell you. I have limited experience with piston systems.

If I were designing one, however, I'd make sure the piston system itself was built to minimize the drawbacks of the concept. It wouldn't be that difficult to do, and I find it unlikely that the builders themselves are ignoring basic physics in their designs.

In other words, the piston system itself can be designed to minimize these effects. I'm sure at least some of the manufacturers are doing that. They have to figure all these forces anyway, so they may as well use the numbers to their advantage for reliability and longevity.

[C4IGrant]

That's exactly what i was thinking.

Now still talking about the same two M4 rifles, one DI the other piston operated, everything else being equal. Knowing that the piston operated rifle would try to unlock the bolt faster than the DI rifle, what features must the piston operated M4 need to SLOW DOWN the initial unlocking of the bolt?

On a DI gun, an angled cam pin shaft would slow this down (like with the LMT Enhanced Carrier). On a piston gun, I do not believe that it would work the same as the carrier is actually pulling the bolt backwards.

So about the only thing I can think of to slow down the un-lock speed would be a heavier buffer. Most piston manufacturers know this and is why they generally us H2 buffers.

One of the real problems with the piston guns (especially the drop in models) is that the gas ports on the barrels are setup for DI. Piston guns do not need that much gas to operate. So this is why piston guns feel like they have so much more recoil than DI guns (generally speaking).


C4