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Powder burn rate VS gas operated rifle

Discussion in 'Rifle Country' started by velocette, Nov 30, 2012.

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  1. velocette

    velocette Member

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    I've an interesting problem. I reloaded 100 rounds of .223 for my AR rifle.
    It has a 20" Shilen barrel with rifle length gas system.
    I used Sierra's recommended accuracy load for the 52 gr match bullets, 23.5 gr of VV N133 powder. (0.5 gr below max) What I get is short stroking. It will cock the hammer & eject the empty but not pick up a new round or lock back. OK, switch to my match load, 69 gr with Rel 15, it cycles perfectly. Also with 55 gr and H 322. Apparently the faster burning N133's pressure curve does not fit well with the rifle length gas system. (The N133 ammo also just barely cycles my RR .223 AR with rifle length gas system)
    So I will reload the 52 gr bullet with slower powder to get proper functioning. I am somewhat displeased in that the N133 & 52 gr Sierra is really accurate but not quite worthy if it doesn't cycle.
    Just interested to learn if other folks have had similar situations.

    Roger
     
  2. MachIVshooter

    MachIVshooter Member

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    N133 is about the same burn rate as IMR4198, which runs great in my ARs.

    That seems like a light load with a 52 gr. Maybe check some other manuals.
     
  3. NWcityguy2

    NWcityguy2 Member

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    VV itself shows a max load of 25gr for N133 and 52gr bullets so you've got some upward mobility if you want to increase pressure. At the end of the day though if it doesn't cycle your gun, it doesn't cylce your gun. Try going up to max load and see if it helps.
     
  4. rcmodel

    rcmodel Member in memoriam

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    And therein lies the problem.

    IMR-4198 is too fast for the .223 to give optimum gas port pressure.

    rc
     
  5. W.E.G.

    W.E.G. Member

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    I have some short-stroking problems with N135 and 77-grain bullets in a rifle-length AR.

    N133, being faster than N135, does not surprise me that it gives short-cycle issues.
     
  6. BCRider

    BCRider Member

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    How does the powder burn rate affect the cycling? It seems to me that it's all just a question of how much gas volume is produced by the burnt powder. There needs to be enough gas volume that the pressure in the bore and piston cylinder is raised to a pressure sufficient to cycle the gun and push the bullet at the right speed. Slow or fast shouldn't seem to enter into it as long as the powder is fully burned before the bullet exposes the gas port. So it would seem that the only thing that fast vs slow powder would do is alter the peak pressure seen when the bullet is just beginning to move.

    Or is there some other factor at work that I'm not seeing?

    If it is just gas volume and the barrel pressure produced by that volume then an action which doesn't cycle fully would be simply due to too light a powder charge. Something that should also show up as a lower than expected muzzle velocity in the end.
     
  7. Art Eatman

    Art Eatman Administrator Staff Member

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    If it cycles with regular ammo, then it must be a problem with pressure from the handload. So, increase the pressure with what's used--or maybe try a different powder.

    Or check around for other folks' accuracy loads and don't be limiited to a book. :)
     
  8. MachIVshooter

    MachIVshooter Member

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    All gas operated semi-autos are sensitive to dwell time. If the burn rate is too fast, the pressure will have peaked early and accelerated the bullet more quickly, resulting in less port pressure and reduced dwell time. Powder that is too slow can result in low or high port pressure and/or excessive dwell time.

    Port diameter and location also affect dwell time, of course.

    This holds true of op rod piston or DI systems. Some designs are more finicky than others, but all are affected by variations in dwell time or port pressure, of which port size, port location, bullet weight, powder charge and powder type (and muzzle devices, like suppressors) are factors.
     
  9. carbine85

    carbine85 Member

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    I use IMR4895 with 69 gr bullets in my Varmit AR15. It works great. It'd in the middle of the chart.
     
  10. BCRider

    BCRider Member

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    Thanks MachIV. That makes sense. At least it does if the "dwell time" is the time between the bullet exposing the port and leaving the muzzle.

    Oddly enough I've seen the same issue occur in a .22 direct blowback pistol. It shot and cycled just fine with 40gn Standard Velocity. But it suffered numerous stovepipe jams and had poor ejection power when it did clear the spent case. Despite the ammo being "High Power" or "High Velocity" I fixed the problem by softening the recoil spring. The reasoning being that the lighter, high speed bullet wasn't in the barrel for long enough to give the slide a decent enough recoil impulse. Near as I could tell the powder used was the same. But your "dwell time" of the bullet in the bore was shorter. So thus the total recoil impulse was less.

    Sort of the same problem but in one case the piston is a piston and in the other the piston is a slide. I just didn't put these two together until I read your explanation.
     
  11. wanderinwalker

    wanderinwalker Member

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    First suggestion: try Winchester 748 with your 52gr match bullets. I'd be really shocked if you have cycling AND accuracy problems. A load about .5gr under max is my go-to 100-yard AR match load. (And I use Magnum primers for this application, mostly because I have a lot of them and normally use this load in sub-freezing temperatures.)

    Second: somebody please explain to me why fast powders like 4198 in .223? Off the top of my head that's faster than 748 and that's the fastest powder I've ever used in .223. H4895 worked better for me and I've burned my share of RL-15 and Varget running my AR-15 too. (Possibly because I normally run bullets of 69gr or heavier?)

    Oh, and accuracy with 748 and 52s, prone, iron sights, slung-up, 100-yards:

    016.jpg
     
  12. NWcityguy2

    NWcityguy2 Member

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    Faster powders are going to be more economical because the charge weight will be lower. Great idea if it cycles your gun. Even better idea in a gun that hand cycles. Using 55gr pills 4198 is going to give close to 100 more loads per pound of powder than Varget. Alliant 2400 is even better. My Mini 14 cycles 4198 just fine.
     
  13. rcmodel

    rcmodel Member in memoriam

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    False economy if it is too fast for the gas system and beats the snot out of the rifle to get it to cycle though.

    rc
     
  14. velocette

    velocette Member

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    Has nothing to do with economy. Had to do with Sierra's recommended accuracy load for their 52 gr match bullet. (have you priced VV powder recently?)
     
  15. MachIVshooter

    MachIVshooter Member

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    That is exactly the definition of dwell time.

    Well, the recoil impulse was less, but not so much because of the duration to bullet was in the bore. In blowback or short recoil operated guns, the slide or bolt has barely started to move when the bullet exits:

    http://www.youtube.com/watch?v=otpFNL3yem4

    It is the inertia created that actually moves the slide. Similarly, the bullet is long out of the barrel on a gas operated gun by the time the bolt unlocks and starts moving back:

    http://www.youtube.com/watch?v=xeeeFxA_9nA

    Case can't start leaving the chamber before the bullet is out of the barrel in any design, or you get a case failure and all that accompanies it, from brass shrapnel and powder residue leavng the ejection port at high velocity to catastrophic gun ka-boom.

    Most likely, these "high velocity" rounds had a lighter bullet. While bullet weight and velocity (and to a certain extent powder charge weight and burn rate) affect recoil impulse, they don't affect it in equal parts. Bullet weight has more effect on free recoil energy, bullet speed has more effect on free recoil velocity. For instance (an extreme example), my .17 Remington with a 25 gr. bullet at 4,245 FPS produces right at 1,000 ft/lbs muzzle energy. My .44 Magnum with a 240 gr. bullet at 1,400 FPS registers 1,044 ft/lbs. They even use similar powder charges (weight, not type). If it were a simple linear equation worked from muzzle energy to determine free recoil, they'd be roughly the same. But it's not, and they're not even close. The .17's recoil impulse in a 7 lb rifle is a whopping 3 ft/lbs. The .44's in a rifle of the same weight is more than double. In the case of your .22, if you were comparing, say, 32 gr. CCI Stingers to normal HV 40 gr pills, the faster Stinger produces more energy but less recoil impulse. Additionally, the Stingers (and other hyper-velocity .22 rounds like the Aguila Supermax) use a slower powder to gain performance in rifle barrels, and when fired out of a handgun, often produce less energy than standard HV ammo.

    In short, what will cause a blowback or short recoil firearm to short stroke is the recoil impulse, not the amount of time the bullet spends traveling down the bore.
     
  16. NWcityguy2

    NWcityguy2 Member

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    Yeah I think most people understand that. But for some posters getting in the last word is really important, even if it is just "It's only a good thing if it works".

    How much powder did you buy and what are you doing with the reloads? You certainly won't be the first person out there to use a semi-auto as a single shot for one reason or another if you really like the load. Contacting Sierra is another option. If this problem is common they may know exactly what advice to give.
     
  17. dvdcrr

    dvdcrr member

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    Anybody have a list of good powders known to work well with rifle length gas systems?
     
  18. wanderinwalker

    wanderinwalker Member

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    I've never tried it, but I think H335 has a loyal following for loading .223 with lighter bullets, and pretty much anything slower up to Varget/RL-15/N140 should work well.

    As for economy in loading, I get it. But I agree with rcmodel that sometimes it's better to use something a little "better" (more suited is perhaps a better term) in some applications. I used to use 55gr FMJs and 748 for "practice" ammo when I started shooting Highpower. That lasted until I realized I was wasting money, barrel life and practice time on ammo that was just "good enough" to hold the 10-ring. Granted, if I was loading for a standard AR or Mini-14, I'd go right back to FMJs and ball powder for that use.

    And my opinion/experience is shaded toward running heavy bullets at full-throttle in .223, hence my fondness for Varget or RL-15.
     
  19. velocette

    velocette Member

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    Well; I've gotten some good advice. First off, I have 8 lb of H335 on my powder shelf so I'll load up the 52 gr bullets with that & it should solve the short stroking due to the too fast N133 in my rifle length gas system.
    Then I'll use up the N133 in my bolt .222 with 50 gr bullets. Should work out ok and I'll use up all my powder efficiently.
    A good days worth of learning and proceeding.
    Thanks all.

    Roger
     
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