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Making AR-Platform Pistols Work

Geissele Automatics URG-I, top and 7.5-inch AR pistol, bottom

I have mixed feelings about these projects, and it’s a mix indeed. I love them. I hate them. Opinions notwithstanding, if you want to shoot AR-platform pistols, here are a few ideas on how to get it functioning.

Geissele Automatics URG-I, top and 7.5-inch AR pistol, bottom for AR-platform pistolsThis one runs fine and shoots well in 5.56 with a 7.5-inch barrel. There is a noticeable difference in case expansion and condition comparing the spent cases from this one and the author’s Geissele Automatics URG-I (top).

The central example here is a 5.56 NATO/.223 Rem. with a 7.5-inch barrel. We’re using this because that shortest configuration seems to attract the most interest. However, it’s also the most problematic. Why is it problematic? It’s too much of a good thing, if high-pressure gas is a good thing. Of course it is! That’s what operates our gas-operated system.

The amount of pressure at the gas port, called gas port pressure, is ultimately the most influential element in system behavior. If there’s not enough, the system doesn’t get enough contained within it for a long enough time to reliably cycle the gun. That’s not usual. What’s most usual is that the gas port pressure is higher than what is needed to cycle the action. The nearer to the chamber the gas port is, the higher will be the port pressure. Port pressure and chamber pressure are not technically the same thing, but they are from the same thing, which is propellant gas.

The highest pressure is just after ignition, just about when the bullet leaves the case neck. From that point, the bullet is moving down the barrel bore, followed by a flooding rush of propellant gas that’s burning all the while. As the bullet moves forward and the gas moves, there’s more volume within the bore to accept this gas, more time for pressure to lower. At the point where the bullet crosses the gas port line, as much gas as can heads that direction and into the gas bloc, then into and through the gas tube. That goes into the carrier key and pushes back the bolt carrier. When the bullet passes the muzzle plane, the lid is off any further pressure building, but it’s been contained fully within the system until that point. It’s essentially sealed up, and the total volume available for the gas to occupy is variable with different systems.

Buffer tube and spring for an AR-15 rifle or pistolThere’s no easy button. Do not install an adjustable gas block. It’s not going to hold up. The main ally we have is all in the back end of the gun: buffer and spring. You need to go heavier on both.

Just get a picture of that in your mind. And now we’ll move it around a little.

A few now-standard numbers: Rifle gas port location is 12 inches forward from the chamber; carbine location is 7 inches; pistol-length location is 4 inches. Dang. And yes, more on port pressure. (Gas port hole sizes vary, and that’s also a variable I can’t really get detailed about here, but it needs to be smaller on a pistol.)

On a rifle-length barrel and system, gas port pressure has dropped radically from what it was just at the chamber. Original Armalite blueprints called for around 12,500 psi port pressure on its new rifle. With our hot-rodded NATO-spec ammo in common use now, it’s running around 18,000 psi. That’s all fine. However, at a scant 4 inches distant, we’re around 50,000 psi with a pistol.

So to make a pistol work, we just need to figure out what to do with nearly three times too much gas. The short answer is we have to learn to live with it—work with it, work around it. All this is happening in milliseconds (0.001 second), and, given the radical speed of the processes, fractional milliseconds are noticeably influential in system behavior.

AR-15 barrel with 4-inch gas tubeThat’s short! With the gas port a scant 4 inches forward, there’s a huge amount of hot, high-pressure gas getting into and through the system.

My Suggestions

  1. Don’t choose 5.56. Another option, such a .300 Blackout, has less pressure and also a bigger bullet to offset some of the velocity and energy loss with 5.56, which are both substantial.
  2. Choose a little longer barrel. Going to a 10.5- or 11.5-inch barrel is an option for a carbine-length system. Huge difference! And since there’s less barrel post-port, the system pressure drops sooner.
  3. Increase buffer weight and spring pressure. Ramping up buffer weight and spring pressure slows bolt unlocking a tad because that combination delays carrier movement. The stouter spring also dampens the carrier so it doesn’t hit as hard.
    Note: There are specialty pistol buffer tubes, which don’t follow any standard, but stick with USGI standard for best options.

Athena Linear forward-venting muzzlebrakeA forward-venting muzzlebrake is most helpful. Blast is vicious. This is an Ultradyne Athena Linear, and it works well.

Another wise trick is increasing extractor tension or grip. Since the case is getting yanked too soon, there’s a little more pressure latent in the case compared with a longer-length system. The extractor can lose its grip because the case is still expanded right at the start of the bolt unlocking. Again, this is more so than in a carbine or rifle. Polishing the chamber helps too, and keep it clean!

Important: Just cutting back load pressure will not necessarily make your AR pistol work better. Then it might not work at all. Since the entire volume of the system is so small, and its operational time is so short, there has to be too much pressure. To a point, at least. That’s the part we have to live with. Longer systems are more flexible and more tolerant of varying-pressure ammo, meaning pressure that varies downward.

Last, I suggest getting a NATO chamber. That’s likely to be the most common in a component barrel, but I see them also with Wylde chambers. The influential differences that matter to this point have to do with throat length, which is also volume. The NATO or 5.56 chamber has the longest throat, biggest volume, and therefore lowest pressure, or highest pressure tolerance, however we want to see it.

Do you own an AR-platform pistol? How does it shoot? What adjustments of aftermarket equipment have you added to your AR-platform pistols? Share your answers in the comment section.

Source: Cheaper Than Dirt

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