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Thread: barrel length, gas system, dwell time, accuracy

  1. #41
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    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.
    Last edited by jmart; 06-01-10 at 16:47.

  2. #42
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    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.
    Last edited by TehLlama; 06-01-10 at 17:14.
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  3. #43
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    Quote Originally Posted by TehLlama View Post
    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.

  4. #44
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    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.

  5. #45
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    Quote Originally Posted by Sundo View Post
    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.
    Really POF and LWRC and accuracy is your goal?
    Shoe horning a a gas piston on the barrel is not conducive to accuracy.

  6. #46
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    Quote Originally Posted by Sundo View Post
    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.

  7. #47
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    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.

  8. #48
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    Quote Originally Posted by jmart View Post
    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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