View Full Version : barrel length, gas system, dwell time, accuracy
Here's a question for you AR gurus:
I'm trying to choose between two ARs that are identical, except for a barrel length difference of 16 or 18 inches. The gas block is in the same location relative to the chamber end, at the midlength position. This means the length of barrel in front of the gas block is 2 inches longer on the 18 inch barrel than on the 16 inch barrel. In turn, this means the dwell time (bullet between gas block and muzzle) is correspondingly longer in the 18 inch barrel.
I've seen charts that say the dwell time increases from 0.018 to 0.023 seconds between a 16 in and 18 in barrel with a midlength gas system, using 55 gr ammo. (I expect to use 77 gr ammo, so I expect the dwell time to be a bit longer.)
My theoretical intuition tells me that the longer dwell time of the 18 in barrel should result in less accuracy than the shorter dwell time of the 16 in barrel, since all else is equal. I notice that the SPR design uses an 18 in barrel with an intermediate length gas block, which puts the gas block further forward than the midlength system.
So my question to AR technical gurus, and those who have experience with the situation above: Are my concerns above valid? Is the accuracy difference measurable (and consistent) in bench rest groups with heavy match ammo? Does anyone have examples of bench rest results that show a clear difference (or clearly no difference)?
I'm trying to learn the results from expert experiences, rather than try to buy both and figure it out on my own. Thanks!
I cannot address the dwell time issue, but can offer to point out that the US DOD chose 18 inch barrels for their 5.56 SPRs, and they are seeking accurately placed shots to 800 meters.
I cannot address the dwell time issue, but can offer to point out that the US DOD chose 18 inch barrels for their 5.56 SPRs, and they are seeking accurately placed shots to 800 meters.
The Mk12 SPRs have an 18" barrel with an intermediate length gas system (between mid length and rifle length), so the dwell time on the SPR 18" barrel with intermediate gas system should be similar to that of a 20" barrel with rifle length gas system or 16" barrel with midlength gas system.
If I get an 18" barrel with midlength gas system, I'll be increasing the dwell time compared to a 18" barrel with intermediate length gas system. I'm wondering if/how much this will reduce accuracy, and what the net effect is compared to a 16" barrel with the same midlength gas system.
longrange18
05-28-10, 14:54
I don't know about accuracy shift but somewhere USMC03 added a post about barrel velocities and he got in to dwell times of different barrel lengths.
He stated that the 16" barrels with a carbine length gas system created a longer dwell time than a 14.5" and would create a rifle with more violent recoil. So I would guess that you could be looking at more recoil impulse with the 18" over the 16".
Found this on USMC03 web site. Carbine vs. Mid-length (http://www.03designgroup.com/technotes/carbine-vs-mid-length-gas-system) Doesn't compare 18" vs. 16" but is a good read for people concerned about gas system lengths.
I don't know about accuracy shift but somewhere USMC03 added a post about barrel velocities and he got in to dwell times of different barrel lengths.
He stated that the 16" barrels with a carbine length gas system created a longer dwell time than a 14.5" and would create a rifle with more violent recoil. So I would guess that you could be looking at more recoil impulse with the 18" over the 16".
Found this on USMC03 web site. Carbine vs. Mid-length (http://www.03designgroup.com/technotes/carbine-vs-mid-length-gas-system) Doesn't compare 18" vs. 16" but is a good read for people concerned about gas system lengths.
My LMT .308 has a carbine length gas system, I really wish they would have gone with mid length, but we will see if they release barrels with a mid length.
Belmont31R
05-28-10, 15:16
The Mk12 uses a RIFLE length gas system not intermediate. The Noveske 18" SPR barrel uses an intermediate length system but its not the correct mil-spec barrel. If you tried to use a Noveske barrel on a clone build the gas block would be under the handguards.
The Mk12 uses a RIFLE length gas system not intermediate. The Noveske 18" SPR barrel uses an intermediate length system but its not the correct mil-spec barrel. If you tried to use a Noveske barrel on a clone build the gas block would be under the handguards.
I stand corrected.
That means the SPR has less dwell time than a 20" AR with the same rifle length gas system. I'm wondering how much that contributes to accuracy.
Belmont31R
05-28-10, 15:32
Not enough (if any) to worry about.
Originally posted by Sundo:
I've seen charts that say the dwell time increases from 0.018 to 0.023 seconds between a 16 in and 18 in barrel with a midlength gas system, using 55 gr ammo.
For starters, your numbers are WAY off. It takes a 55 grain bullet less than 1 millisecond, that’s 0.001 seconds, to travel the entire length of an 18” barrel. It takes less than 0.9 milliseconds, 0.0009 seconds, for that 55 grain bullet to travel the entire length of a 16” barrel. The difference between the dwell times of a 16” barrel and an 18” barrel that both have a mid-length gas system will be approximately 0.06 milliseconds; that’s 0.00006 seconds.
55 grain bullet from 18” barrel
http://www.box.net/shared/static/tf13gf27xk.jpg
55 grain bullet from 16” barrel
http://www.box.net/shared/static/vol5emaez6.jpg
Originally posted by Sundo:
My theoretical intuition tells me that the longer dwell time of the 18 in barrel should result in less accuracy than the shorter dwell time of the 16 in barrel, since all else is equal.
A. I’ve never seen a single study showing a causal relationship between an increase in the amount of time that it takes the bullet to travel from the gas port to the muzzle and a decrease in accuracy in an AR-15.
B. “All else” is not equal.
.....
I was always under the impression that the benefits of a midlength were:
Less pressure put on the BCG, allowing a longer service life
Less violent extraction
Less felt recoil
I was always under the impression that the benefits of a midlength were:
Less pressure put on the BCG, allowing a longer service life
Less violent extraction
Less felt recoil
ditto: +1 me too.
rifle gas is even better.
Thomas M-4
05-28-10, 17:13
I was always under the impression that the benefits of a midlength were:
Less pressure put on the BCG, allowing a longer service life
Less violent extraction
Less felt recoil
Rifle length gas system does that but more so than the intermediate system.
The rifle length gas system and rifle length buffer system makes for the softest cycling system. [The way it was originally designed]
Rifle length gas system does that but more so than the intermediate system.
The rifle length gas system and rifle length buffer system is the makes for softest cycling. [The way it was originally designed]
This is why I don't understand the carbine length gas system on the LMT.
Thomas M-4
05-28-10, 17:21
This is why I don't understand the carbine length gas system on the LMT.
On the 5.56 MRP I think they have 2 different gas system lengths carbine and mid length if I remember correctly. The .308 MRP use of a carbine gas system I don't know for fact but I imagine that LMT has there Bolt carrier correctly ported and timed for use in the carbine gas system.
On the 5.56 MRP I think they have 2 different gas system lengths carbine and mid length if I remember correctly. The .308 MRP use of a carbine gas system I don't know for fact but I imagine that LMT has there Bolt carrier correctly ported and timed for use in the carbine gas system.
I have their .308, and I think the gas system is the one thing I don't like about it, the rest is golden.
FrankRochester
05-28-10, 21:46
So a 14.5" with mid gas should really begin to bring the recoil down more towards the 20" barrel then?
So a 14.5" with mid gas should really begin to bring the recoil down more towards the 20" barrel then?
No, it's milder than a carbine but is still 5.5 inches shorter than the rifle. Lot's more felt blast. Recoil on any is not an issue. It's muzzle rise and blast that effects your shooting with an 5.56 AR system.
Are my concerns above valid
No, I don't believe they are.
What setup are you currently using, and what size groups are you getting?
I don't mean to hijack the thread, but one quick question:
Are there any advantages of using a midlength gas system in an 18" barrel compared to using the new intermediate gas system?
I don't mean to hijack the thread, but one quick question:
Are there any advantages of using a midlength gas system in an 18" barrel compared to using the new intermediate gas system?
i dont think so. the intermediate system seems to be the proper ratio of between 60-65%. it's an improvement over both previous options.
it's an indicator, not a rule:
14.5 carbine - 52%
14.5 mid - 65%
16.1 carbine -47%
16.1 mid - 59%
18 mid - 53%
18 intermediate - 64%
18 rifle - 69%
20 rifle - 63%
Barrel length has nothing to do with accuracy.
C4
Robb Jensen
05-30-10, 10:10
I don't mean to hijack the thread, but one quick question:
Are there any advantages of using a midlength gas system in an 18" barrel compared to using the new intermediate gas system?
The intermediate was made for a 18" barrel, if it's available in the barrel you want to get I'd highly recommend getting it.
The midlength will shoot well with an 18" barrel but recoil well be a tad more harsh and the pressure a little more than ideal, midlength was really meant for 16" barrels and if you look at KACs version it's even longer than the other ArmaLite system used by everyone else and it shoots softer better still.
Thank you very for the information guys.
The intermediate was made for a 18" barrel, if it's available in the barrel you want to get I'd highly recommend getting it.
I'm actually trying to figure out if I should have my 18" (Noveske mid gas length SPR upper) cut down to 16", or to just fogetaboudit and keep it as is.
The Mk12 SPRs have an 18" barrel with an intermediate length gas system (between mid length and rifle length), so the dwell time on the SPR 18" barrel with intermediate gas system should be similar to that of a 20" barrel with rifle length gas system or 16" barrel with midlength gas system.
No, military SPRs use rifle-length gas systems.
For starters, your numbers are WAY off....
Sorry, I was listing milliseconds, not seconds. My mistake.
The table I've been referring to is here:
http://ar15barrels.com/tech/pressure-time.gif
For the "proper length gas system" (14.5 barrel, carbine length gas system; 16 inch barrel, midlength gas system; 18 inch barrel, intermediate gas system; 20 inch barrel, rifle gas system) the dwell time ranges from 0.181-0.206 ms. The 16 in barrel with carbine gas system or the 18 inch barrel with midlength gas system have the two longest dwell times: 0.248 and 0.234 ms, respectively. An 18" with intermediate gas system has a dwell time of 0.181 ms.
The difference between the dwell times of a 16” barrel and an 18” barrel that both have a mid-length gas system will be approximately 0.06 milliseconds; that’s 0.00006 seconds.
While that difference of 0.053 ms is small compared to our normal sense of time, that is a 30% increase in dwell time compared to the 18" barrel with intermediate length gas system. I'm assuming the operation of the gas system introduces disturbances into the barrel motion that are not there before the start of dwell time. Thus, longer dwell times should inflict more disturbance on the barrel while the bullet is between the gas port and muzzle. Is this not reasonable?
I'm interested in a SPR/DMR type of rifle, so that last bit of accuracy is important to me, if it is measurable.
...
A. I’ve never seen a single study showing a causal relationship between an increase in the amount of time that it takes the bullet to travel from the gas port to the muzzle and a decrease in accuracy in an AR-15.
I haven't seen formal studies, either. I have seen magazine article reviews of 14.5" carbine length gas system rifles having better accuracy than very similar 16" carbine length gas system rifles. This data, by itself, isn't conclusive. So, I'm hoping somebody has tried a side-by-side test.
B. “All else” is not equal.
.....
I'm considering a rifle that is otherwise identical, except one version has a 16" barrel and another version has an 18" barrel. The rest of the upper receiver (including gas system length) is identical.
Barrel length has nothing to do with accuracy.
Correct.
Dwell time has nothing to do with accuracy for practical terms, disregard that. Dwell time is important in being able to consistently cycle the bolt, but doesn't affect internal or external ballistics in any noticeable way.
As much as you'd like to believe that those minutae might matter: they won't. You're not going to get below 3/4MOA 10-shot groups with an semi AR-15, and that's with a well developed load for it.
If you're comparing a true apples to apples 16" and 18", and weight isn't a factor just get the 18" for the added barrel length and be done. From all your other information, those comparisons aren't really oranges to oranges. To really even get much use out of a precision rig like this I'm assuming you could do with the extra muzzle velocity, so 18" might fare better.
Dwell time has nothing to do with accuracy for practical terms, disregard that. Dwell time is important in being able to consistently cycle the bolt, but doesn't affect internal or external ballistics in any noticeable way.
So, what is the relationship between dwell time and reliable operation?
So, what is the relationship between dwell time and reliable operation?
You need enough dwell time to charge the gas system to ensure reliable cycling. Too much dwell (i.e., too much gas) stresses the system and is hard on parts.
Dwell and gas port diameter have to be optimized to get the correct volume of gas. If you have short dwell you need to open up the port some. If you have excessive dwell, then you'd need a smaller diameter port to regulate the amount of gas being sent back to the carrier. It all goes hand in hand.
Although it sounds as though gas port location/dwell time have no measurable effect on accuracy in the AR15's direct gas action, the theoretical question isn't so far-fetched. According to noted Garand-smith Clint Fowler, gas timing can have a significant effect on the accuracy in the Garand:
http://clintfowlerrifles.com/2008/07/23/the-m1-garand/#more-13
I became aware of it after building a rear lugged M-1 with a Krieger barrel for a customer in Washington State. On the test bench it shot 2¼ inches. For a Krieger barrel this was not acceptable so I set the gun aside with a feeling of frustration. I knew the gun had been built with care. I knew the barrel was first class. It had to be something else. A week later the hair-top computer spun out the answer. It was timing. The bullet was still caught in the rifling when the op-rod started to move rearward. It set up barrel vibrations, which affected accuracy. This explained all those guns with good components, which did not shoot well. They could all have been made to shoot good if a way had been found to delay the op-rod function until the bullet just cleared the muzzle...
The solution was an adjustable gas plug. A plug with a threaded bore with two setscrews mounted inside. By adjusting them outward the volume was increased. More volume, more time needed to fill it. Time measured in parts of a millisecond. To say this solution worked is an understatement. It was the missing link, the last rear hurdle...
Please note that this is Mr. Fowler's claim, not mine.
Although it sounds as though gas port location/dwell time have no measurable effect on accuracy in the AR15's direct gas action, the theoretical question isn't so far-fetched. According to noted Garand-smith Clint Fowler, gas timing can have a significant effect on the accuracy in the Garand:
http://clintfowlerrifles.com/2008/07/23/the-m1-garand/#more-13
Please note that this is Mr. Fowler's claim, not mine.
Please note that the AR-15s being discussed do not have an operating rod.
.....
You need enough dwell time to charge the gas system to ensure reliable cycling. Too much dwell (i.e., too much gas) stresses the system and is hard on parts.
Dwell and gas port diameter have to be optimized to get the correct volume of gas. If you have short dwell you need to open up the port some. If you have excessive dwell, then you'd need a smaller diameter port to regulate the amount of gas being sent back to the carrier. It all goes hand in hand.
OK, that make sense. A 16" mid-length gas system will have lower gas pressure and shorter dwell time than a 16" carbine-length gas system. So by this logic, the mid-length gas must have a larger gas port to function correctly. Does this hold true with available barrels?
Please note that the AR-15s being discussed do not have an operating rod.
.....Please note that I specifically said the the effect seems to have "no measurable effect on accuracy in the AR15's direct gas action...", but it " can have a significant effect on the accuracy in the Garand." You even quoted me.
What gave you the impression that I felt that the AR-15s being discussed DO have an operating rod?
As I stated: "the theoretical question isn't so far-fetched." It has been an issue in other gas-operated weapons, specifically the Garand, according to Fowler.
The OP asked a legitimate question, and I think that he got his answer from other posters. I simply pointed out that the phenomenon had been an issue in another gas-operated rifle design, according to a well-respected gunsmith.
OK, that make sense. A 16" mid-length gas system will have lower gas pressure and shorter dwell time than a 16" carbine-length gas system. So by this logic, the mid-length gas must have a larger gas port to function correctly. Does this hold true with available barrels?
I will defer to other SME's to answer this. IIRC gas port diameter on carbine length systems should be around .062". I don't know if mfg's adjust this diameter for 14.5" vs 16" barrels, it may not matter, but they do open them up when building SBRs (due to the short dwell).
I also don't know if they adjust port diameters for mid-length vs carbine-length gas systems, sorry. But it is universally understood that by extending the gas system length, the port experiences lower pressure compared to a carbine-length positioned port, and that's why they run a little more smoothly. Of course, to get effective smooth operation, the port has to be sized correctly, otherwise you could still end up being under- or over-gassed.
Note that gas port diameters are very different depending upon rifle length gas systems, carbines and SBRs (with minimal dwell).
Please note that I specifically said the the effect seems to have "no measurable effect on accuracy in the AR15's direct gas action...", but it " can have a significant effect on the accuracy in the Garand." You even quoted me.
What gave you the impression that I felt that the AR-15s being discussed DO have an operating rod?
As I stated: "the theoretical question isn't so far-fetched." It has been an issue in other gas-operated weapons, specifically the Garand, according to Fowler.
So basically, your post about the effects of operating rods is completely irrelevant to the subject matter of direct impingement gas sytems being discussed in this thread and yes, the OP's question is far fetched.
With the direct impingement gas system there are no moving parts involved until after the bullet has left the barrel. That’s the beauty of the direct impingement system on the AR-15. Dwell time in the AR-15 direct impingement gas system is a non-issue for accuracy in an AR-15. If it was, according to the OP's "theory," 24" barreled AR-15s with rifle length gas systems, with their more than 10" of barrel from the gas port to the muzzle, should have the worst accuracy of all AR-15s. All of the 10-shot groups pictured below were fired from a 24" barreled AR-15 with a rifle length gas system from a distance of 100 yards.
The OP could do far more to improve his accuracy by choosing a quality barrel with a length suited to his mission needs, choosing the gas system length that has demonstrated the greatest reliability in that length, purchasing a large amount of quality ammunition and start practicing, than by fretting over the imagined significance of dwell time on accuracy in an AR-15.
http://www.box.net/shared/static/lcjet2zjr7.jpg
http://www.box.net/shared/static/9zxgg2bsdy.jpg
http://www.box.net/shared/static/1npzvb68ax.jpg
http://www.box.net/shared/static/e2fc1vhxz2.jpg
So basically, your post about the effects of operating rods is completely irrelevant to the subject matter being discussed in this thread and yes, the OP's question is far fetched...
You can qualify my post any way you wish, I merely pointed out that some gas operating are affected, and yes the Garand uses an op-rod, but both systems use gas to drive a piston. As Fowler pointed out, it's not the op-rod per se, but the volume of gas that is required before the piston starts to move. Both systems have pistons, but in one system, the Garand, that piston might move before the bullet leaves the barrel, according to Fowler.
In the AR15-style direct gas action, the piston (which is integral to the bolt) moves later. The direct gas system effectively increases the volume of the gas system, much like Fowler increased the volume of the Garand's gas system, if Fowler is to be believed.
Yes, the Garand's op-rod is long and heavy, and it's premature movement has greater potential to adversely affect accuracy, but if Fowler is to be believed, once the volume of the Garand's gas system is increased, the piston/op rod won't move until the bullet leaves the barrel.
The issue is gas volume, not the op-rod.
If you feel that my post is off topic, and "irrelevant", fine, you're entitled to your opinion. Perhaps you should report my outrageous, "completely irrevant" post to a moderator. Maybe you should even report the OP, because you feel that his question is so "far fetched."
Instead, you argue with me over a point which I agreed to in the very first sentence of my first post -- it's not an issue in the AR15-style direct gas action.
Knowledge and scrappin' to boot on this forum. I gotta disagree though, being a benchrest guy, you really can't discredit any part of a system from affecting accuracy - doesn't matter if it moves or not. Your ultimate accuracy potential ALWAYS ends with barrel vibration and timing.
I appreciate all of the informative responses.
FYI, I am trying to choose between two otherwise very similar off-the-shelf rifles. I am NOT putting together a custom rifle, so "choosing the best [custom] barrel" is not an option. The only option I'm giving myself is choosing between a 16" barrel or an 18" barrel, with a mid-length gas system, on an otherwise identical rifle design.
Even though a DI system has no moving mechanical parts, I was wondering if the diversion of the gas itself might disturb the barrel harmonics and timing, and thus affect shot accuracy. Judging by the nice groups that are presented from a 24" barrel, rifle length gas system rifle, that is not necessarily the case. Is that because the DI gas operation has negligible effect on barrel harmonics/timing, or is it because the DI gas operation effect is consistent enough that the disturbance is the same every time, imparting a consistent effect on the bullet?
How much more significant does the match between barrel length and gas system length become if a gas-piston operating system is used? If the piston's movement doesn't start until after the bullet has left the barrel, will there be any effect at all?
I appreciate all of the informative responses.
FYI, I am trying to choose between two otherwise very similar off-the-shelf rifles. I am NOT putting together a custom rifle, so "choosing the best [custom] barrel" is not an option. The only option I'm giving myself is choosing between a 16" barrel or an 18" barrel, with a mid-length gas system, on an otherwise identical rifle design.
Even though a DI system has no moving mechanical parts, I was wondering if the diversion of the gas itself might disturb the barrel harmonics and timing, and thus affect shot accuracy. Judging by the nice groups that are presented from a 24" barrel, rifle length gas system rifle, that is not necessarily the case. Is that because the DI gas operation has negligible effect on barrel harmonics/timing, or is it because the DI gas operation effect is consistent enough that the disturbance is the same every time, imparting a consistent effect on the bullet?
How much more significant does the match between barrel length and gas system length become if a gas-piston operating system is used? If the piston's movement doesn't start until after the bullet has left the barrel, will there be any effect at all?
The questions you are posing, IMHO, are really getting down to the theoretical, and don't have a lot of relevance to the practical aspects of deriving maximum accuracy potential from an AR platform. Hope that doesn't come off as "dickish", that was not my intent. But it was meant to convey we've entering the "paralysis from analysis" mode and we're disregarding the more relevant aspects of what contributes to accuracy, which was your original line of questioning.
Are you opposed to identifying you two candidates and asking for input and pros/cons of each choice? Reason I suggest this COA is that I believe barrel quality and sighting system employed will have a far greater impact on accuracy than whether or not your 18" option is configured mid-gas vs rifle-length, etc.
Even though a DI system has no moving mechanical parts, I was wondering if the diversion of the gas itself might disturb the barrel harmonics and timing, and thus affect shot accuracy. Judging by the nice groups that are presented from a 24" barrel, rifle length gas system rifle, that is not necessarily the case. Is that because the DI gas operation has negligible effect on barrel harmonics/timing, or is it because the DI gas operation effect is consistent enough that the disturbance is the same every time, imparting a consistent effect on the bullet?
I am not an AR-15 engineer, but my understanding is that:
a. The gas port, gas block, gas tube are in the same place every shot so have exactly the same barrel harmonics every shot.
b. Movement from gas escaping from the gas port does not begin until the bullet has left the barrel.
How much more significant does the match between barrel length and gas system length become if a gas-piston operating system is used? If the piston's movement doesn't start until after the bullet has left the barrel, will there be any effect at all?
Barrel length seems generally irrelevant in either system, assuming that the bullet is stabilized (1" of bbl probably does not stabilize the bullet for a 1000 yd shot).
However, for whatever reason, DI guns are much easier to get to shoot accurately than piston guns.
The questions you are posing, IMHO, are really getting down to the theoretical, and don't have a lot of relevance to the practical aspects of deriving maximum accuracy potential from an AR platform. Hope that doesn't come off as "dickish", that was not my intent. But it was meant to convey we've entering the "paralysis from analysis" mode and we're disregarding the more relevant aspects of what contributes to accuracy, which was your original line of questioning.
No offense taken. And yes, I am in a state of analysis paralysis. I tend to remain in that state for a while before making an expensive decision. Shooting is a hobby for me, not a profession, so there's very little negative consequence to analysis paralysis when it comes to big spending decisions.
Are you opposed to identifying you two candidates and asking for input and pros/cons of each choice? Reason I suggest this COA is that I believe barrel quality and sighting system employed will have a far greater impact on accuracy than whether or not your 18" option is configured mid-gas vs rifle-length, etc.
For the purpose of stimulating further discussion, I do hesitate to identify the specific models I'm considering. I will re-evaluate that decision as the discussion progresses. I will, however, provide the following details:
The model(s) I'm considering are NOT primarily designed for utmost accuracy. They have chambers designed for reliability, so they're not .223 chambers; they are 5.56 NATO or similar. The barrel is medium weight, cold hammer forged. The remaining rifle components are high-quality (but not necessarily highest match quality) AR parts, including a two stage trigger. Based on reviews, I expect around 1-1.25 MOA accuracy at 100 yds with heavy match ammo (2-3 MOA with light, cheap ammo).
Of the characteristics described above, I suspect a Wylde chamber might improve accuracy by 0.25 MOA and a true .223 chamber a tad more, at the expense of reliable cycling under adverse (dirty, hot) conditions. I suspect a match quality heavy, 20-24 inch barrel might improve accuracy another 0.25 MOA. All other improvements (including hand-loading) might improve accuracy another 0.25 MOA. Beyond that, I see very little room for improvement. I suspect it is very hard to get an AR-15 that consistently groups smaller than 0.25 MOA, although I'm sure it is possible. Are these reasonable expectations of accuracy vs. other tradeoffs?
As for optics, I consider this a separate issue, since optics can be changed. I assume a decent 3-9x scope will be plenty to produce accuracy at the rifle's potential at 100 yds. In practice, I'm looking at a 3-9 or 2.5-10 scope without parallax adjustment. With this setup, I hope to achieve consistent, first-round hits of 18" round targets out to 600 yds with heavy match ammo. At 700-800 yds, I expect 2-3 shots may be required for a hit, with rapid deterioration beyond that. Of course, this depends on a stable shooting platform, mild winds, and a good shooter (which I hope to become). This is my expectation, but I am considering what factors might improve or limit performance further.
From the discussions I've read so far (all of which I highly appreciate, BTW), I'm getting the impression that going from a 16" to an 18" barrel will have a less than 0.25 MOA effect on 5 shot groups at 100 yds, with a larger effect at longer ranges. If I'm shooting at 600 yds with heavy match ammo, which barrel length (using a midlength gas system) is more likely to produce smaller, consistent groups, if there is any measurable difference at all? (Assume the supersonic limiting range is 750-800 yds.)
Again, thanks to all for a fascinating discussion. I enjoy analysing these types of issues almost as much as actually shooting the rifle I will purchase as a result of these discussions. I am very tempted to actually buy and try both versions of the rifle I'm considering, but I will have to temporarily suspend my rules against reckless spending to do that.
Your accuracy requirements aren't all that great (18" @ 600 equates to 3 MOA), but you keep mentioning accuracy as if it's a key requirement. So based on that I'm going to offer up my opinion on getting you into a rig that will far surpass 3 MOA, more like .75 MOA. This also assumes this rig won't see high rates of fire like a carbine endures at a training class, and it also assumes you reload and can devote some time to developing a good long range load.
Wylde or other match variant chamber. While 5.56 NATO can be accurate, it won't be as accurate as a match-stye chamber. White Oak Precision and Compass Lake are two good vendors who can help you out here.
If going out to 600 yards and beyond is expected, then I'd opt for the 18" barrel, assuming it was high quality (Krieger, Schneider, PacNor). Velocity will be your friend. Note that these will be SS barrels, not CM chrome-lined.
I wouldn't worry about the ML gas on the 18" tube, I think the extra velocity will trump any theoretical accuracy degradation experienced by the extra dwell.
If barrel vendors mentioned above don't offer 18" tubes, consider going with a 20" tube. They might not come in mid-weight profile though, you might have to adopt a HP bull profile under the HGs (these are heavy buggers).
Whatever you go with, free floating will be a necessity. You're probably already aware of this, just didn't want to overlook an assumption.
If my assumptions above were wrong, and 3 MOA is sufficient, then I'd just go with something like a Denny's Operator. These are good barrels, and while I don't have many rounds out of mine, what I do have during a shakedown session at 100 yds with 55 grain ball sure came in well below 3 MOA. With proper match bullets I fully expect to get under 1.5 MOA but I haven't had the time to load up any of my 75 Hornadays that still sit in the box.
Beckman - that is pretty good insight on the op-rod dwell time/action issues, and certainly helped my understanding, though Molon's right about that being outside the scope of this argument.
To squeeze out maximum accuracy at 100yd, might as well run a heavy 16" and drop the gas system altogether - but it sounds like you have an opportunity to take it out to 600+ meters - In which case a 16" or 18" (in middy, intermediate, or in the case of the 18 even rifle length gas system). While you can get more than adequate consistency out of a 10" barrel, velocity will make the biggest difference, hence why longer barrels. Between otherwise like barrels, at 100m there's no difference between a 16" and 18".
[ETA] Jmart did a better job explaining this...
You can hit 18" targets at 500m with a rack grade M4 barrel using good ammunition, and that's not even floated. You should be able to easily maintain MOA accuracy out to half a click with a floated 16" barrel using match/HPBT ammo, With an 18 or 20" and an adequate pile of practice ammunition, hits on 18" steel is doable beyond 600m, for sure; just be prepared to spend a bit more on ammunition for the good stuff out there.
Good 3-9x or 2.5-10x glass will be a big part, so find one you like and a mount that works, and you'll be set there. NF 2.5-10x would be great, or if money's tight, the new Vortex PST 2.5-10x (it's very large/heavy, but the larger objective isn't bad for range only use). I personally run a Leupold Mk4 2.5-8x36mm ITMR unit on an 18", and I've been pretty happy with it.
I'd say any medium or thicker contoured SS barrel, properly floated will give you the results you're after - 16" or 18" will do, and gas system won't be that relevant. Any quality barrel with a good BCG in spec, with good ammunition will cycle just fine. A Noveske or GTS Operator will get that done with a 5.56 chamber, and any of the high end (Douglas, Krieger, CLE, WOA) options with slightly tighter chambers will probably yield great results. Handloading (75 and 77gr OTM bullets) stands to give you even better results, though digging through Molon's prolific and informative posts will be a huge help here - Hornady and BlackHills seem to be perennial favorites.
If you're looking at opening up the old budget, I'd just go ahead and pick up a high end SS barrel, go a bit over budget on the optic and mount; get a top end trigger (Jewell, Wilson TTU, Timney, or a Geisselle) and start looking at either reloading, or getting top end match rounds. There's a multitude of great options, and staying MOA with match ammunition is quite do-able.
Beckman - that is pretty good insight on the op-rod dwell time/action issues, and certainly helped my understanding, though Molon's right about that being outside the scope of this argument.
[...]You're cetainly entitled to your opinion, but obviously I disagree. The OP asked, "Are my concerns above valid?" My aswer was that it wasn't an issue in the Stoner-style direct gas, but it might be an issue in other gas operated rifles, specifically the Garand.
I find it ironic that arguing with both you and Molon over the appropriateness of my post has consumed much more bandwith than did my actual post. At this point, I suggest that we either drop it, discuss it via PM, or as I said to Molon, if you think that my post is so far "outside the scope of this argument" that you need to mention it in your post specifically adressed to me, then I suggest that you report my post to a moderator.
Okay, it's time to reveal my possible choices.
My current top two choices are the LWRC M6A3 or the POF P415, both in either 16" or 18" barrel, both in 5.56/.223. I will also consider DI designs, but leaning toward piston. I've also read about the LWRC SPR which should become available "mid 2010".
The few reviews I've seen show the M6A3 14" doing .7 MOA groups and the M6A2 16" doing 1.0 MOA groups, bench rest, heavy match ammo. I've read a review of the P415 18" doing .7 MOA. Those are very few points of comparison.
Ideally, I'd like to be able to make consistent hits at 750 yds with heavy factory match ammo, on an 18" target. That's about 2.5 MOA at 750 yds. To get that, I estimate I need to get .75 MOA groups at 100 yds. Is this realistic, or too tight? I'm guesstimating that .75 MOA at 100 yds grows to 2.5 MOA (that's MOA, not inches) a 750 yds.
Based on this, I think I'll be happy with a rifle that has the velocity to reach 750 yds supersonically (Black Hills 77 gr HPBT), with .75 MOA accuracy at 100 yds. It sounds like I just might achieve that with either the M6A3 or P415. What do you think?
I'm starting to think the 5.56 chamber is a limiting factor on accuracy, as DPMS rifles with true .223 chambers seem to be more accurate. I favor the 5.56 chamber for reliability, but want to get as much accuracy as I can without going custom.
Thomas M-4
06-03-10, 17:28
Okay, it's time to reveal my possible choices.
My current top two choices are the LWRC M6A3 or the POF P415, both in either 16" or 18" barrel, both in 5.56/.223. I will also consider DI designs, but leaning toward piston. I've also read about the LWRC SPR which should become available "mid 2010".
The few reviews I've seen show the M6A3 14" doing .7 MOA groups and the M6A2 16" doing 1.0 MOA groups, bench rest, heavy match ammo. I've read a review of the P415 18" doing .7 MOA. Those are very few points of comparison.
Ideally, I'd like to be able to make consistent hits at 750 yds with heavy factory match ammo, on an 18" target. That's about 2.5 MOA at 750 yds. To get that, I estimate I need to get .75 MOA groups at 100 yds. Is this realistic, or too tight? I'm guesstimating that .75 MOA at 100 yds grows to 2.5 MOA (that's MOA, not inches) a 750 yds.
Based on this, I think I'll be happy with a rifle that has the velocity to reach 750 yds supersonically (Black Hills 77 gr HPBT), with .75 MOA accuracy at 100 yds. It sounds like I just might achieve that with either the M6A3 or P415. What do you think?
I'm starting to think the 5.56 chamber is a limiting factor on accuracy, as DPMS rifles with true .223 chambers seem to be more accurate. I favor the 5.56 chamber for reliability, but want to get as much accuracy as I can without going custom.
:rolleyes: Really POF and LWRC and accuracy is your goal?
Shoe horning a a gas piston on the barrel is not conducive to accuracy.
Okay, it's time to reveal my possible choices.
My current top two choices are the LWRC M6A3 or the POF P415, both in either 16" or 18" barrel, both in 5.56/.223. I will also consider DI designs, but leaning toward piston. I've also read about the LWRC SPR which should become available "mid 2010".
The few reviews I've seen show the M6A3 14" doing .7 MOA groups and the M6A2 16" doing 1.0 MOA groups, bench rest, heavy match ammo. I've read a review of the P415 18" doing .7 MOA. Those are very few points of comparison.
Ideally, I'd like to be able to make consistent hits at 750 yds with heavy factory match ammo, on an 18" target. That's about 2.5 MOA at 750 yds. To get that, I estimate I need to get .75 MOA groups at 100 yds. Is this realistic, or too tight? I'm guesstimating that .75 MOA at 100 yds grows to 2.5 MOA (that's MOA, not inches) a 750 yds.
Based on this, I think I'll be happy with a rifle that has the velocity to reach 750 yds supersonically (Black Hills 77 gr HPBT), with .75 MOA accuracy at 100 yds. It sounds like I just might achieve that with either the M6A3 or P415. What do you think?
I'm starting to think the 5.56 chamber is a limiting factor on accuracy, as DPMS rifles with true .223 chambers seem to be more accurate. I favor the 5.56 chamber for reliability, but want to get as much accuracy as I can without going custom.
As long as bullets are properly stabilized, MOA is MOA, meaning. .75 MOA at 100 yards should hold to .75 MOA at 750. That would grow to about a 5.6" group assuming there's no keyholing/stabilization issues.
The .223 chamber would offer the caomparatively better accuracy, but note that you shouldn't run any NATO-spec ammo of any weight through a .223 chamber, you're just asking for trouble with popped primers, higher pressures, case head separations, etc. Better off to select a Wylde chamber which has a longer freebore and shallower leade or a NATO chamber altogether.
If max accuracy is your goal, I too would recommend you skip the piston and go with DI. It's proven to work and I can't think of any long range AR rig that features a piston. And in competition circles, while Service Rifles are required to use DI, match rifles have more latitude in configurations and I can't think of any HP Match Rifles that features a piston. And ARs have supplanted piston-driven M1's in SR for the most part, they're just easier to shoot and offer stellar accuracy using DI and good, heavy weight BTHPs. Just saying.
My apologies for anyone who felt misled by the revelation that I'm compromising accuracy with a piston operating system. I'm willing to give up that last 0.25-0.50 MOA of accuracy to get a piston system. My original question was about the effect of mid- vs. intermediate-gas system length on an 18" barrel AR. The discussion diverged in a couple different directions, which I didn't mind.
Having said that, the LWRC M6A3 series has an adjustable gas system that includes a "CLOSED" or "OFF" setting, that closes off the gas feed to the piston. This disables cycling, making it a single-shot, manual cycling rifle, much like a bolt action.
So, here's a new (but related) question.... If I close off the gas cycling on the M6A3 and fire it as a manually cycled single shot, how much accuracy (if any) can I expect to gain? My understanding is that the movement of the heavy piston in a piston system degrades accuracy more than the movement of the gas in a DI system. Can I expect anything like a 0.25 MOA improvement in accuracy if I fire the M6A3 in manual cycled mode?
Again, thanks to all for a fascinating discussion.
As long as bullets are properly stabilized, MOA is MOA, meaning. .75 MOA at 100 yards should hold to .75 MOA at 750. That would grow to about a 5.6" group assuming there's no keyholing/stabilization issues.
That's sounds great in theory, but I am a little skeptical of how well that holds up in practice. In the few reviews I've read where they tested accuracy at different distances, the longer distances generally produced larger MOA group sizes, not just larger inch group sizes with the same MOA. In two reviews at different distances, the MOA size increased about 11-33% per 100 yds increase in distance. If I assume a 20% increase in MOA size per 100 yrds, starting with 0.75 MOA at 100 yds, I get 2.5 MOA at 750 yds, for a group size of 18.5 inches. That might be pessimistic. If I assume a constant increase of 0.2 MOA per 100 yds (instead of compounding 20%), I get 2 MOA at 750 yds, for a group size of 15.375 inches.
I can see the logic of assuming MOA size remains the same at any range. I suspect that's true if the variation is due entirely to angular dispersion. I think it's the variation in muzzle velocity that causes increasing MOA with range. That's just a theory though.
The .223 chamber would offer the caomparatively better accuracy, but note that you shouldn't run any NATO-spec ammo of any weight through a .223 chamber....
Already understood. The reminder, for safety, is appreciated. I prefer the 5.56 chamber for better cycling reliability under dirty conditions.
If max accuracy is your goal, I too would recommend you skip the piston and go with DI...
I'm looking for max accuracy within the limits of a piston-driven AR with 5.56 chamber. So, I realize I won't be able to get 0.25 or 0.50 MOA accuracy. I'm trying to figure which off-the-shelf options will give me 0.75 MOA accuracy with heavy match ammo. From the few reviews I've read, there are documented examples of the LWRC M6A3 14" and the POF P415 18" piston ARs producing 0.75 or better groups. I'm wondering if the M6A3 16" or 18" will also give 0.75 MOA accuracy. I'm suspicious that the increased barrel length in either rifle, and the increased dwell time in the 18" rifle, might compromise accuracy.
I know in bolt action rifles, you can experiment with hand-loading to find the powder load weight that gives best accuracy. Does this work on ARs as well?
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