SBRs are stupid

So I’m talking mostly about AR pistols.

I’m no ballistics expert, but here’s my understanding: You have x amount of propulsion and n amount of mass. The idea is to propel that mass down a confined tube with maximum efficiency.

More mass takes more propulsion to get to a given velocity.

Now we’re not dealing with whatever values we like. There are not only constraints to the values available to us there is also a limitation of pre-made combinations of these values.

Ideally, the propulsion you have available is completely expended at the same point your mass leaves the confined tube. For morons, the powder is burned at the moment the projectile leaves the barrel. Not before, not after.

So for off the shelf bits, this leaves you with rifle rounds designed for rifle-length barrels and pistol rounds designed for pistol-length barrels. Now, I’m not smart in these things but I’m mostly certain that your average off the shelf 5.56 ammo is designed for about an 18 inch (maybe 20) barrel. You fire this round through a 6 inch or 10 inch barrel and you’ve blown most or half of your propulsion out the barrel to no effect. You get a loud noise, shitloads of unburned powder leaving the barrel and a projectile that is moving far slower than intended.

To my estimation this is not much different than firing an off the shelf 9mm round out of a 20 inch barrel. The propulsion has expended itself before your projectile reaches the end of the barrel resulting in a slower than optimal projectile.

I could be entirely wrong about all of this and I’m quite certain someone will tell me I’m completely full of shit. If you want a short barrel, you should be using a pistol ammunition.

I totally understand the idea of a short rifle. It gives you a forearm for stability but is short enough to maneuver in tight spaces.

For me, that means pistol caliber carbine. Like God intended. Not this silly AR pistol shit.

/begin flamewar

ArmedGeek

14 Comments

  1. You’re not entirely wrong. On the other side many gun company’s make rifles in pistol calibers, 9mm etc. they are gaining in popularity. I carry an AK in an SBR, perfect as a truck gun.
    Also, I loved your martini post from awhile back, mixing booze with more fucking booze. Indeed.
    CIII

  2. You’re right from a technical standpoint, of course. But just look at ’em! They’re Badass! And I can use the same ammo I use in my carbine!

  3. I’m with you. I have a T/C Contender single shot pistol that I’ve bought many different caliber barrels for over the years. One that I had was in .223/5.56mm. Huuuugggge fireball coming from the business end of that one. I spent many hours on the loading bench trying different recipes to minimize this effect. All to pretty much no avail. Finally decided that it wasn’t worth the effort and sold that one………

  4. Yes and no. To get the maximum performance out of any bullet the powder load needs to be tuned to the barrel using the correct powder and quantity of that powder for a particular gun. I am not a fan of rifle cartridge SBR’s mainly because I am a cheep bastard and rifle rounds are more than twice as expensive to produce. I prefer a pistol cartridge in an AR pistol. A 10mm 155gr in my G40 (6″ barrel) is running 1570 fps. In my AR pistol with a 11.5″ barrel the same load gets up to 1963 fps. The biggest drawback of the AR pistol is that it is a very dirty running gun and needs cleaning after 200 or so rounds through it.

  5. You are correct.
    However, under current laws, it is perfectly legal to make a pistol at home, including an Ar15 pistol.

    Also, it is perfectly legal to buy an 80% AR lower and finish it yourself.
    And it is legal to put a stabilizing brace ( not a stock) on it.
    So a 30 round standard capacity mag full of fmj, and /or hollow point varmint , and/or penetrator rounds is completely legal in a semi automatic package for less than a Glock or brand name 1911.

    Yes there is a huge fireball and concussion.
    Feature, not bug.

    If Spicy Times arrive, it’s a very handy tool too have in the get home bag.

  6. about 400 fps gain on some of the high pressure pistol round but only 200 fps out of the low pressure rounds out of a rifle barrel.

  7. Life is compromise. I WANT a small light minimal recoil accurate rifle I can employ from my car if necessary. I WANT it suppressed and very accurate and to have great stopping power and lots and lots of bullets in the mag. I WANT to not have it on a .gov list so my neighbor can’t Swat me with a false red flag report. I WANT it to cost $500.00, so if it ever gets used to defend me n mine, I can replace it.

    Getting all that in one package is un-possible.

    But you CAN finish an 80% lower and buy a 10″ upper and use a pistol tube and arm brace. That gets me great flexibility and maneuvering, without getting the gun and me on a list.

    30 round mags fill the need for lots of bullets in case I get caught up in a crowd of attacking antifa.

    300 blackout with a heavy expanding boolit yields good stopping power.
    No legal way to suppress it without the tax stamp, so there is a compromise.

    Muzzle flash, yeah the fireball and concussion will be a factor, but for range fun and in the event of having to use it for self defense, that may be a feature, not a bug.

  8. Anything this complicated actually requires engineers. There are three types: 1) those that ignore physics and put rifle rounds in a pistol arrangement because someone wants it or it is cool, 2) those who say barrel length, powder rate of burn, amount of powder, and projectile mass need to be balanced to get optimal performance, and 3) those engineers who do not understand the trades.

    It is fun to be in the first class for a little while, to learn what the limits really are. Second class is a thankless place to live. Third class will do whatever anyone tells them without saying “bulls*t”.

    Fun blog posting.

  9. Re pistol rounds in carbines:

    From a physics viewpoint, even though the propellent may have ceased burning, the bullet will continue to accelerate as long as the force due to the gas pressure exceeds the frictional force between the barrel and bullet. There is probably no carbine or rifle barrel so long that the bullet does not continue to accelerate along its whole length.

    • ….somewhat complete burn -and- minimal muzzle flash,
      are the basis of SuperPerformance from Hornady.

      It’s an interesting subject, maintaining chamber pressure
      over a long(er) barrel.
      Ballistics by the inch….addresses the other side of this equation!
      😉

  10. This is why you have different burn rates for different barrel lengths, projectile mass and bore volume.

    If the powder isn’t burning, and smokeless powder is a PROPELLANT, not an explosive, how does the expanding gas which propels the projectile expand?

    • Not a mechanical engineer, but this sounds reasonable to me. The propellant does burn and causes the gas behind the bullet to expand sue to heat. This should, theoretically, create a well of high pressure gas behind the bullet. Until the pressure is equalized with the gas impinging on the business end of the barrel, the bullet should be pushed down the barrel. There will be some frictional component that would have to be overcome, but I would guess that the pressure in the gas behind the bullet would far exceed the frictional component and the impinging gas pressure for some considerable length of barrel. Sorry if this is incorrect, but this seems logical to me.

  11. Former USArmy Armorer / former gunsmith / Retired Forensic tester:
    9MM 124 grain FMJ NATO ammo continues to accelerate through 16 – 17 inches of MilSpec barrel, then starts to slow. Muzzle Velocity (IIRC) was 1850 – 1950 fps. Most, but not all, pistol cartridges act similarly. Most .45 ACP will start to decelerate at about 12 inches. .22 LR tops out at 20 – 24 inches, depending on brand. .32 ACP Fiochi FMJ (!) tops at 22 inches at 1900 fps.

    Try it and test it. If you roll your own, you can tailor most ammo to anything.

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