"Safe" max pressure out of AR 15?

Discussion in 'Handloading and Reloading' started by gifbohane, Apr 18, 2021.

  1. gifbohane

    gifbohane Member

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    I have been trying to find the max (without pushing it) pressure, as measured by feet per second, out of the end of a 16 inch barrel of an AR 15 that shoots 223 and 556.

    I have chronoed a 16 and a 20 inch barrel and, of course, I noticed a difference in FPS out of each one. The longer giving me the most FPS.

    Is the max really a function of the nature and quality of each individual chamber and barrel? Yesterday one of my shots registered over 3100 FPS a significant increase over all the others and it got me to wondering what the max FPS was, if it can be generalized.

    Again I am NOT interested in pushing the FPS just being careful.
     
  2. edwardware

    edwardware Member

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    There's no such thing. Velocity is a result of pressure, and chamber and bore dimension and finish, powder burn rate, etc.

    Yes, and the same load in two different barrels of the same length might be 100fps different just for bore finish and dimension.
     
    Last edited: Apr 18, 2021
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  3. gifbohane

    gifbohane Member

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    Edward--

    So you are saying that there is no way of measuring pressure produced by each individual round...outside of a laboratory?

    Or another question "What is the FPS number really telling me?" Anything of value? A small indicator of pressure in that barrel?
     
    Last edited: Apr 18, 2021
  4. Havok7416
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    Havok7416 Member

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    While you don't need a lab, you absolutely need the proper equipment to do it. Velocity does not translate to pressure without other variables involved.
     
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  5. MEHavey

    MEHavey Member

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    There is no way outside of a transducer for determining MAX pressure achieved during the firing process.
    As hinted at above, pressure/velocity is dependent upon a multitude of factors -- not the least of which least is
    integrating the TOTAL area under the pressure curve, ....not the Max.

    223Comp2.jpg

    CAUTION: The above includes load data generated by calculation in QuickLOAD software based on a particular powder lot, the assumption the primer is as mild as possible, and assumptions about component, chamber and gun geometry that may not correspond well to what you have. Such data should be approached by working up from published starting loads. USE THIS DATA AT YOUR OWN RISK. Neither the writer, The HighRoad, nor the staff of HighRoad, nor QuickLOAD's author nor its distributor assume any liability for any damage or injury resulting from use of this information or information derived from it.
     

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    Last edited: Apr 18, 2021
  6. Charlie98

    Charlie98 Member

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    Think of velocity as a puzzle piece... one of many that complete a picture of a handload in a particular firearm. There are other pieces to look at, unless FPS is your end goal and nothing else matters, including pressure or accuracy nodes.
     
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  7. kalielkslayer
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    kalielkslayer Contributing Member

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    Velocity is a byproduct.

    One powder may only give you 2,900 fps yet be at max pressure while another may give you 3,000 fps and be well below max pressure.
     
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  8. Slamfire

    Slamfire Member

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    Let me tell you how we service rifle shooters did it.

    We loaded rounds, tested them, took them to matches. If the round blow or pierced a primer, we cut the charge by half a grain. Shot some more, and we kept on cutting a half grain at a time till the primers stayed in place, and stopped piercing.

    The more you shoot, the more surprises you will find.

    It was always surprising how ammunition loaded in 70 degree weather would be over pressure in 90 F weather.

    Loads that worked perfectly in my Wilson match barreled NM AR15's (Wilson are button rifled barrels) blew primers in my Krieger cut rifling barrels. I had to reduce my loads by 1.5 grains for the Krieger barrels. Krieger barrels are tighter, that is all.

    Depending on the bolt manufacturer, your bolt will crack its lugs between 10,000 and 30,000 rounds. Stoner built the bolt to last 6000 rounds, and added a 2:1 safety factor. So you can expect a AR15 bolt to crack a lug at some point, and the bolts that are made out of more advanced materials, and shot peened, will last longer. None of them will last forever.

    https://www.ar15.com/forums/ar-15/High_round_count_AR_M4_s__over_100_000_rounds__and_how_they_have_handled_on_our_range/118-677135/?
     
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  9. hdwhit

    hdwhit Member

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    But you ARE pushing/exceeding maximum pressures!

    Are you mad?

    In the circumstances, it is a cogent question.

    There is no reliable method (outside a fully equipped ballistics laboratory, which I doubt you have) for correlating velocity with pressure in the absence of other data (i.e. what powder, case, primer, are you using), yet you report neither powder type, powder charge nor projectile weight.

    So, on what basis are you selecting powders in this "velocity quest"?
     
  10. Demi-human

    Demi-human Member

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    No, it’s just kind of a rude one.:)

    CFE223 easily bests 3,100 FPS in a sixteen inch barrel at published charges.

    Here are some loads that beat 3,300 in a 24”. All at less than SAMMI specified maximum pressure.
    https://hodgdonreloading.com/reloading-data-center?rdc=true&type=54
    There’s more if we wish to step outside the accepted “normal” for an AR. But for an average loss of 25-35fps per inch of barrel reduction, that puts the velocity right in line.

    Would he still be insane if he’d never chronographed them at all?

    If all of them were over expectations that would be a sign of too high a load.
    If just one is over, that is a sign of a difference with that single cartridge. Maybe in the loading process, perhaps how it was loaded by the rifle(set back), maybe in the bullet. It might make me look at my loading procedures a little finer.

    But with just one round being different, I don’t think there is much to go on.

    Have you chrono’d the difference in American Eagle ammunition? I bet yours is closer...;)

    My personal perceived average is 2900 fps from a sixteen inch barrel with typical factory loads, taking from myself, shooting acquaintances and range reports from other shooters. You’re not that far off.
     
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  11. NMexJim

    NMexJim Member

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    Are you just interested in attaining max pressure safely or is there another driving reason such as identifying an accurate load?
     
  12. AJC1

    AJC1 Member

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    Velosity is a relative measurement of relative pressure with the same components in your rifle. Using exactly the same components but increasing powder of the same type will give you more speed and pressure under the same conditions. Using mixed brass your observations are basically useless.
     
  13. Bfh_auto

    Bfh_auto Member

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    If we aren't given projectile weight or primer we can't guess if he's over pressure.
    I have loads that run over 3100 in a 16" barrel. They aren't over book max either. But they are 40 grain bullets.
    I got this from his post.
    (Should I be worried that my one shot was way over velocity of the others. I don't want to blue my gun up.)
     
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  14. gifbohane

    gifbohane Member

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    Read the first sentence (and my last sentence) in my post and then apologize for the snarky response.
     
    Last edited: Apr 19, 2021
  15. gifbohane

    gifbohane Member

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    NMex Not looking for max or anything close to it. Just trying to find out if there is a number that I should be steering clear of. For example I guy that I respect told me that 1200 FPS with a 9mm cartridge is getting hot, so I steer well clear of that number.
     
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  16. jebova2301

    jebova2301 Member

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    You can't just base it on X feet per second. Different powders have different burn rates. Fast burning powders will spike up to maximum pressure, and then fizzle out before the bullet has moved very far. Slow burning powders will still be combusting as the bullet is traveling down the barrel.

    Velocity is related to pressure, sure. But pressure changes based on volume. Think of your barrel as a variable height cylinder. If you use an ultra fast powder, you get an immediately high pressure, but the powder is done burning near instantaneously. As the bullet travels down the bore, the volume of your cylinder is increasing, meaning pressure is decreasing. If you have a slower burning powder, it may never hit that same peak pressure as the fast powder, but it will still be burning and expanding as the bullet travels down the bore, this keeping the pressure higher for a longer amount of time.

    A simple way of seeing this is to look at the data hodgdon provides. It shows pressure and velocity. Some powder will show a higher pressure, but lower velocity than other powders.
     
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  17. gifbohane

    gifbohane Member

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    Thanks for a very clear explanation. Appreciate it...
     
  18. denton

    denton Member

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    There have been some responses that I think are well founded, and well presented. Without trying to stir the pot too much, here are some slightly contrarian views that I think are correct, based on my many measurements of pressure and muzzle velocity:

    Pressure, minus barrel friction, engraving force, and a minor term for energy diverted to spin, uniquely determine muzzle velocity for a given barrel length and bullet mass. If you know those things, you know muzzle velocity. Period.

    Chamber geometry, cartridge length, leade, bullet mass, type of powder, powder charge, and chamber temperature (and maybe some things I've momentarily neglected) determine pressure. All those things drive pressure, and pressure (minus the appropriate terms listed above) drives muzzle velocity.

    It is true that MV is determined by the area under the pressure curve. You can put time on the horizontal axis, and figure MV from the change in momentum, or you can put distance on the horizontal axis and figure MV from the work done accelerating the bullet. In the second case, the end points of the distance are fixed at the breach and at the muzzle. The only way to get more or less area under the curve (for a particular length barrel) is to raise or lower the peak pressure. Experiment confirms what you would expect, given that information: Peak pressure is very highly correlated with muzzle velocity for a given combination of bullet, barrel, powder, etc. Another way of saying that is: If you have one of those numbers, you can do a good job of predicting the other.

    Sometimes, as you increase powder charge, you will reach a point where adding more powder does not increase MV. For a "book" recipe, so long as you are operating below that point, if you are at "book" MV for a given load, then you are also at "book" peak pressure.

    The rub is that published MVs are usually for some barrel length other than the one you have. There are some rules of thumb for correcting for that, or QuickLoad will help you figure out how to compensate.
     
    Last edited: Apr 19, 2021
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