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

  1. #31
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    Quote Originally Posted by jmart View Post
    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?
    Last edited by pinepig2; 05-31-10 at 14:15.

  2. #32
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    Quote Originally Posted by Molon View Post
    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.

  3. #33
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    Quote Originally Posted by pinepig2 View Post
    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).

  4. #34
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    Quote Originally Posted by beckman View Post
    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.














    Last edited by Molon; 05-31-10 at 16:30.
    All that is necessary for trolls to flourish, is for good men to do nothing.

  5. #35
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    Quote Originally Posted by Molon View Post
    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.

  6. #36
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    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.

  7. #37
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    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?

  8. #38
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    Quote Originally Posted by Sundo View Post
    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.

  9. #39
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    Quote Originally Posted by Sundo View Post

    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.

  10. #40
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    Quote Originally Posted by jmart View Post
    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.

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