AR-15 Bullet Travel vs Bolt Travel

Discussion in 'Main Message Board' started by skeeljc, Nov 30, 2019.

  1. skeeljc

    skeeljc Silver $$ Contributor

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    Looking for information on AR-15 action timing. What I want to know is the relationship of the bullet position in the barrel vs the bolt carrier position in the receiver.

    I know the bolt unlocks and begins to move rearward just after the bullet passes the gas port (15" for a rifle length gas tube). I am running a 24" barrel and wonder if the bullet clears the barrel before the buffer contacts the end of the buffer tube.
     
  2. garandman

    garandman Bolt Gun Bodacious Gold $$ Contributor

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    I don't have the answer to your question though I am sure you will get very strong opinions that contradict each other As to whether or not the bullet exits the barrel before the bolt bottoms out on the buffer.

    I can say this discussion on the whole is what caused me to dump all my semi auto precision rifle platforms for bolt guns.

    Other people whom remain convinced serious accuracy is possible from a semi auto said you just have to Extend your follow through to seemingly absurd levels to get accuracy from the platform.
     
  3. mike mccormick

    mike mccormick Gold $$ Contributor

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    Seems that it would depend on the buffer arrangement. A heavier buffer, or a more heavily sprung buffer will slow the travel quite a bit. If the buffer is optimal for the rifle, it should never bottom out in the tube
     
  4. urbanrifleman

    urbanrifleman Site $$ Sponsor

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    Tubb shot the AR 10 for a couple seasons. I asked him what he thought.

    He said something like it was great when it worked. Super great. But it was too hard to keep consistent.

    He never mentioned the accuracy.
     
  5. skeeljc

    skeeljc Silver $$ Contributor

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    I am running a JP LMOS bolt carrier with a JP Silent Capture Spring. Trigger is a Geissele High Speed National Match. It is very smooth. Gun weighs 13.5 pounds. It stays on target when fired. It will do 1/2 MOA five shot groups at 100 yards about 80% of the time.
     
  6. dfrosch

    dfrosch Silver $$ Contributor

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    If your buffer bottoms out in the tube, your AR is over gassed. The idea behind an adjustable gas block is to constrict the gas flow so that it moves the buffer just enough to cycle the action, and no more.
     
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  7. Fast14riot

    Fast14riot Silver $$ Contributor

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    I shoot 24" barreled 6.8 ARs, dwell time with a 110gr bullet going 2750 is going to be long, however port pressure will also be much lower. Even though the gas system can still be 15,000psi in a 12" rifle length gas system, your bullet is travelling very close to muzzle velocity already and the carrier sure isn't travelling that fast. Maybe somewhere around 300-400fps. So, the bullet should have exited the barrel before the carrier has even fully extracted the case.
     
  8. Doug Beach

    Doug Beach Silver $$ Contributor

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    The bolt / carrier / buffer / etc weighs many, many times what the bullet weighs. When the gas port is passed by the bullet, the bullet is moving over 2000 FPS, and the operating system is essentially motionless. The movement of the carrier, prior to bullet exit, is probably some relatively small fraction of an inch. All it needs, prior to the gas pressure dropping, is momentum.
     
    Last edited: Nov 30, 2019
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  9. garandman

    garandman Bolt Gun Bodacious Gold $$ Contributor

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    When the bullet exits the barrel, the whole system instantly depressurizes. The vast majority of the Gas will take the path of least resistance.... out the muzzle. The buffer will need to have travelled 90% (ish) of its path b4 the bullet exits the barrel. . Even A 5oz buffer is nothing compared to 15,000 psi.

    An adjustable gas system does *not* guarantee against the buffer bottoming out. It merely reduces felt recoil.
     
  10. Doug Beach

    Doug Beach Silver $$ Contributor

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    Just for clarity, I may be misunderstanding.. are you saying that the bolt is unlocked, and most of the way to the rear, before the bullet exits?
     
  11. garandman

    garandman Bolt Gun Bodacious Gold $$ Contributor

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    Honestly, I dont know. I dont know how anyone would determine exactly where the bolt is when the bullet exits the bbl.

    My point is simply this - the bolt will have to have travelled *most* of its path b4 the bullet exits the muzzle and the system instantaneously depressurizes ... simply because of the laws of physics re: gases.

    I am unaware of the exact position of the bolt as the bullet exits. But the bolt rotating & cycling while the bullet is still in the bbl will negatively impact accuracy as compared to bolt guns.
     
  12. Doug Beach

    Doug Beach Silver $$ Contributor

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    If the bolt opened, before the bullet exited, you would blow every case. The bolt carrier has begun to move, but has not yet unlocked the bolt. It has achieved sufficient velocity that its momentum overcomes the resistance of the spring.
    If I recall correctly, the piston on an M14 has moved about 1/8” to the rear, before bullet exit. Its momentum finishes the job.
     
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  13. Rodney

    Rodney Silver $$ Contributor

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    How could the bolt remove a cartridge case that's still under pressure? I think the bullet must be clear of the barrel before extraction can occur. Just like a short stroke piston system, momentum does the extraction once pressure has dropped. The carrier might be moving before the bullet leaves the barrel,for there is a small distance the carrier must move to unlock the bolt. I'm no expert, but I did sleep in a holiday inn express. Lol, seriously though I do shoot ar type rifles a lot and that's my guess how that system works.
     
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  14. Rodney

    Rodney Silver $$ Contributor

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    I was to slow on my post Doug.
     
  15. garandman

    garandman Bolt Gun Bodacious Gold $$ Contributor

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    When the bullet exits the barrel, the sytem totally depressurizes, With the gasses taking the path of least resistance.. out the muzzle. and a semi auto can no longer function.
     
  16. Rodney

    Rodney Silver $$ Contributor

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    Hmm, then how does a blowback or delayed blowback work if pressure is required?
     
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  17. garandman

    garandman Bolt Gun Bodacious Gold $$ Contributor

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    I've already explained that 3 times. Because 90 plus percent of the travel has already occurred and momentum will finish the rest of the job and/or because the buffer is already at its full length of travel.

    And as I stated my only real interest in this discussion is to note the inherent inaccuracy of the semi auto platform.. .. that its action cycles while the bullet is still in the barrel. I don't really care exactly where the action is in its travel, simply that it is traveling while the bullet is still in the barrel. That movement *will* be seen down range, on the target.
     
    Last edited: Nov 30, 2019
  18. 46and2

    46and2

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  19. Doug Beach

    Doug Beach Silver $$ Contributor

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    Since you aren’t a believer in momentum..
    Let’s say that the bullet averages 2500 FPS, over the ~5” past the gas port. So, it takes it 1/6000th of a second to exit the barrel, after passing the gas port. In order for the bolt group to move 1” in that time, it would have to average 500 FPS. What mechanism or structure would bring a 1lb item in excess of 500 FPS to a halt, before it exited the butt, and possibly your shoulder?
     
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  20. garandman

    garandman Bolt Gun Bodacious Gold $$ Contributor

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    False. Read my actual posts.

    Believe whatever U want re: bolt / bullet. I dont do internet arguments.

    Have a nice night.
     

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