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  1. #1
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    Jul 2008
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    Default 45acp velocity from a 16" rifle question, help settle an argument. pressure causes

    I got into a little argument with a range buddy today on the velocity of a 45acp when fired from my Oly arms AR45 with 16" barrel.
    He said "That bullet should be fly'en out of that 16" barrel"
    I said "No, the fast burning Tightgroup powder is gone in the first few inches and friction would start to slow the bullet so it should not be much faster than a 1911".
    He said "The bullet is long gone before the pressure drops and causes friction to slow the bullet".
    I said "The Oly is a blowback gun so the bolt and carrier would start to move back the same time the 230g bullet moves forward. The presure would be 0 as soon as the gas can leak around the bolt and with no presure driving the bullet friction would take over".
    Any thoughts? If not I will have to move a chronograph up on my huge list of "want to buys" and test for myself.
    Thanks, Tim

  2. #2
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    Dec 1969
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    Well.. couple of things.. first off , just because the powder quits burning, doesn't mean pressure is at normal atmospheric.. for that to happen the powder would have to finish burning and the resultant gas would have to cool to air temp.. you getting any muzzle flash at all? Muzzle flash is just hot gasses. Thompson SMG has barrel lenght 10.5 -11 inches and list military ball ammo at 930 fps, 1911 has a 5 inch barrel and the same ammo is listed at 820 fps.. the HK USC Carbine with 16' barrel is also 930 fps with Winchester 230 gr and 1075 fps with Cor Bon 230 gr.

    I would say that with standard factory 230 grain ball , you are both half right.. it probably gets a little over 930 somewhere shortly after 11 inches and then slows back down a little and leaves at 930 fps.... however this is not true for 185 gr.. Corbon 185 gr exits at 1400 fps..

  3. #3
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    Even a revolver, that loses pressure quickly, gains higher velocity with a longer barrel. Using the same cartridge in a 2 1/2" barrel, and a 6" barrel, you will see gains in velocity in the longer barrel.
    I don't see how the pressure can be loss. If there is no pressure after leaking around the bolt, there is no pressure to drive the bullet through the rifiling. And you should have a stuck bullet in the bore. Unless the bullet has already exited the bore. It all happens very quickly.
    If a man has nothing greater to believe in than himself, he is a very lonely man.

    I reckon so. I guess we all died a little in that damn war.

    And in that time when men decide and feel safe to call the war insane, take one moment to embrace those gentle heroes you left behind.
    Quote Originally Posted by MEJ1990TM View Post
    Well, all right. Maybe just this once.

  4. #4
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    Your pressure does not "drop to 0" due to early movement of that blow back bolt carrier..... If so, there would be ZERO muzzle blast, ZERO sound at the bullet leaves the barrel of your carbine. I am sure you get some action out at that muzzle when the bullet leaves the barrel...quite enough pressure for the cuts compensator on a Thomson to help with muzzle climb on full auto.

    You do have to expect the law of diminishing returns with LONG carbine barrels on pistol caliber weapons. There is a limit to the extra speed you get...but it takes a REALLY LONG barrel in all calibers but .22LR before the projectile starts to slow down in the bore. Also guys who think they can load up "special carbine loads" to get super hi-vel bullets speeds are wasting their time
    in most cases. If your load is a HiVel pistol round...it will be a good HiVel carbine load as well.

  5. #5
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    Jul 2008
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    Well crap, I might be wrong on this one. My argument was based on a slow, low pressure round in a blow-back AR. After thinking a bit more, I can see that the forces comming back with the bolt may be enough to keep it closed enough to allow acceleration through out the entire 16" bullet travel. I use a 8oz buffer and the bolt/carrier weigh at least 9oz more, then you add the spring and that is a very heavy load to push back. I will add that the rifle is VERY quiet and the muzzle blast seems small, it is not much louder than a super-sonic 22 round. It is interesting that the Thompson is about 100fps faster than the 1911.
    Thanks for the replys, Tim

  6. #6

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    In my 44-40 I gain about 200fps between my pistols and my rifle. In my 45 Colt and 357 Mag. I gain almost 400fps. Iíve never checked it between my 1911 and my Thompson but the way it makes the steel dance Iím sure it is hitting harder.
    Your powder choice and bullet weight will vary the numbers but Iím sure you are getting more out of your AR than you think.

  7. #7
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    Plus, you have to remember Tim, once that bolt starts moving rearward, you still have a plug in that chamber that is holding the pressure. That expanded case. And the bullet is probably long gone before that case is fully out of the chamber and ejected. Again. All of this happens very quickly.
    I once saw a slow motion video of a semiauto pistol being fired and if I remember right, the bullet left the muzzle, before the slide was all the way back. Damn. Wish I'd have saved that video now.
    If a man has nothing greater to believe in than himself, he is a very lonely man.

    I reckon so. I guess we all died a little in that damn war.

    And in that time when men decide and feel safe to call the war insane, take one moment to embrace those gentle heroes you left behind.
    Quote Originally Posted by MEJ1990TM View Post
    Well, all right. Maybe just this once.

  8. #8
    Join Date
    Dec 1969
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    5,382

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    I load .45 Super for my 16.5 inch Enfield .45 ACP carbine. I haven't chrono'd the latest loading, but I'm now shooting 230 grain JHPs to about 1400 fps. No pressure signs, but I might stop there. With the right action and powder, you definitely can improve on .45 ACP ballistics.
    Always looking for interesting 7.62x25 Tokarev and 7.63 Mauser cartridges!!!
    Member: International Ammunition Assoc. (IAA), European Cartridge Research Assoc. (ECRA). Ask me about membership!

  9. #9
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    Quote Originally Posted by natimblue View Post
    I got into a little argument with a range buddy today on the velocity of a 45acp when fired from my Oly arms AR45 with 16" barrel.
    He said "That bullet should be fly'en out of that 16" barrel"
    I said "No, the fast burning Tightgroup powder is gone in the first few inches and friction would start to slow the bullet so it should not be much faster than a 1911".
    He said "The bullet is long gone before the pressure drops and causes friction to slow the bullet".
    I said "The Oly is a blowback gun so the bolt and carrier would start to move back the same time the 230g bullet moves forward. The presure would be 0 as soon as the gas can leak around the bolt and with no presure driving the bullet friction would take over".
    Any thoughts? If not I will have to move a chronograph up on my huge list of "want to buys" and test for myself.
    Thanks, Tim
    You are both partially right, and thus, partially wrong.
    Consider the simple physics of it as thus:
    Assuming the handgun round is potimized to develop velocity in a 5 or 6" barrel, since there is still pressure when the powder burns out, there will still be force pushing on the bullet in the longer barrel. That force will likely increase velocity SOME, how much depends on the exact load's internal balistics of the pressure curve. It IS dropping after burnout, BUT, it is dropping long before burnout anyhow. Peak pressure in the cartrige case is developed shortly after ignition, with a bullet blocking or just barely opening up a small, tightly confined space (cartrige case), not later on. As the bullet heads down the tube, volume behind it opens drastically, gunpowder burn has to try to keep this pressurized while the bullet keeps increasing the area to pressurize, pressure falls.

    Change that load to a slower burning powder, one that displays significant muzzle flash and delivers disapointing velocity in a short barrel, will likely show a serious increase in velocity from a carbine's longer barrel. Instead of powdfer burnout 2 or 4 inches into bullet travel, you get burnout 10 inches or so into travel, with significantly extended time/distance ahead of higher average pressure driving it. Peak pressure may have occured several inches into travel down the barrel, same peak but flatter curve, higher average pressure for the trip down the barrel.

    Now, on the bolt movement, keep in mind, the vast difference in mass between the bullet and the bolt/recoil mechanism.
    The 230gr .45 bullet weighs a fraction of a pound, and has, for a close number, 19,000 psi against it.
    The bolt itself, may weigh a pound, 30x as heavy, will react 30x slower, now add a spring that must be overcome (2 springs actually, the recoil spring and the hammer spring, and the hammer spring is significant due to the leverage it has on the hammer (difference in pulling back the bolt while the gun is cocked (hammer out of the way) and when the hammer is down, definitely notable dif.)). The chamber is, for all practical intents and purposes, sealed until the cartrige case actually leaves the chamber, not while it is extracting. Time is so short for "leakage", that it is effectively nonexistant while the slightly loose case is in the way.
    Once that mass starts to move, inertia keeps it going, but starting it's movement rearward, takes a lot of force, and a lot (relatively) of time.
    The bullet is long gone before the case gets ejected, before barrel pressure reaches "0".

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