Gun Gear|Reviews

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 pistols
This 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 pistol
There’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 tube
That’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 muzzlebrake
A 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.

 

The Mission of Cheaper Than Dirt!'s blog, The Shooter's Log, is to provide information—not opinions—to our customers and the shooting community. We want you, our readers, to be able to make informed decisions. The information provided here does not represent the views of Cheaper Than Dirt!

Comments (15)

  1. Acquired a 7.5 “ Spikes Tactical pistol. Installed a forward venting flash hider and went to the range.
    I could not get the pistol to cycle. It came with a heavy buffer. I tried different ammo without success. Finally purchased an adjustable gas port. Closed it down and began opening until the bolt would lock back on a single round. Opened a slight bit more to guarantee cycling. Pistol then would function perfectly with any ammo. Wow. $ 50 for a cure. Ultimately I replaced the barrel with a 10.5” variety as the fireball was too large and the noise was horrendous. Sith a carbine length gas system, I needed to adjust the gas post significantly and now it is almost full open. Probably do not need an adjustable port for a carbine system but after the frustration dealing with the 7.5” barrel, I would not have a pistol without an adjustable gas port. With the 10.5” barrel, the Spikes is a pleasure to shoot and 100% reliable.

  2. I have an IWI Galil Ace Pistol in 5.56mm, 10″ barrel. Works off the AK-47 functionality and has fired 40 gr, 50 gr and 55 gr. Gun really likes 55 gr. Hornady Spire Point. Shoots 1″ groups at 100 yards with velocity at the mid to high 2,300’s compared to 2,500 for the Fiochi 50 grain with Hornady V-Max. Massive blast with the Linear muzzle device but is actually quieter (relative) than the stock IWI muzzle brake. Pistol performs flawlessly so far with approximately 500 rounds fired. Really heavy with the folding stock. Mostly steel construction and handy in the truck with the stock folded or deployed. I would buy the 300 B.O. if doing it over. I now have a 30 round hot 22 Hornet (actually very hot). Is this a suitable self defense gun. You decide.

  3. I put together a 8.5 inch pistol, using a no-name stripped receiver. I added a Manticore muzzle brake, Geisele Maritme bolt catch, Yankee Hill Machine enhanced trigger guard and ambidextrous loop backing plate, flip up BUIS and a Firefield red dot with a laser built in. The kit and lower were less than $400, cheap enough that I could add a few neat accessories. The company must have everything right, the pistol runs every thing from Federal xm193 to the cheap steel case ammo. I have a CMMG 22 conversion kit and it works perfect in the pistol. Short length, light weight, and accurate at ranges that would be a stretch for most handguns. Great truck gun or camper gun.

  4. Good information, but you may have missed some very good options to enable an AR pistol to cycle properly.

    Mid-Length Gas Systems: I use the Faxon 11.5″ Gunner 5.56 barrel which has a Mid-Length gas system. This allows for me to use the same components that I use in my AR 223 Wylde 16″ barreled rifle.

    I disagree in part with your conclusion concerning adjustable gas blocks. They may fail on pistol length systems, but many are proven on carbine, mid and rifle length gas systems.

    Use a Adjustable Piston Gas system such as the Superlative Arms offering. The Superlative Arms piston system is extremely reliable and I know of well used SBRs using this product that have over 15,000 rounds downrange.

    Given more time I might be able to add more to this discussion.

  5. Could you line up the gas block with the gas port than slide the gas block forward just a tad. Wouldn’t that let less pressure go into the gas tube and decrease pressure on the bolt? Just a thought.

  6. I have a Springfield AR Saint pistol. Probably have 600 rounds though it
    with no failures of any kind. It’s a well built pistol and you
    Can tell Springfield put a lot of thought into it.
    Deadly accurate at 25 to 50 yards 2-3 inch groups at 100 yards and
    I’ve rang steel at 200 yards.
    The only additions I’ve made is to add the Sig iron sights, left over from my Sig 516, a bipod mount, and a Sig Romeo red dot. I am very pleased with this pistol. At the same time I was extremely impressed by Maxim Defense’s PDX. Talked to the reps at the NRA connvention and handled the pistol. To say it’s a masterpiece would be an understatement.

  7. I have nothing but good luck with the Olympic arms pistol that put the buffer system above the pistol
    receiver. Wolf steel case even functions with no jams. These pistols have no buffer tube and Olympic Arms closed up a few years ago.

  8. I have a shorty pistol using a Law Tactical folding stock with a Barking Spider on the end of a Wylde barrel, and I have grenaded one buffer while getting unwanted double and triple taps. I found this rundown very helpful, as when I tried cutting back pressure, it stopped working all together. now I have a better understanding of what I need to do. Thank you.

  9. Another thing that helps is to use an M-16 /full auto style bolt carrieras it has a little more mass than an AR-15 carrier.

  10. 5.56 in a pistol is all wrong. That caliber is all about velocity, and a short barrel takes away the one thing it has. Might as well shoot .22 WMR or .22 LR. I have two AR pistols, one in 9mm, which is my favorite gun, and one in 7.62×39 which is the non boutique version of 300 BLK. The 9mm is pure blowback, and the x39 has a carbine length gas system which works reliably.

  11. Nicely done sir! Informative, concise, and very well written.
    I really liked the point about moving from a 7.5″ bbl to a 10.5″ one. As you said, “carbine length”. Which is what this mess is REALLY designed to be. Not this hacked up piece of crap that is useful for basically nothing but maybe some plinking,but only if your not picky how it preforms.
    I see people shooting them at the range often, yet I have not once seen anyone that was happy with their performance.
    My opinion? Dump it and buy a AR-15! And maybe even a REAL pistol to go with it! Time to add a smile back to the fun of firing an AR firearm.

  12. It’s 4 a.m. so maybe my brain isn’t working. Are you suggesting that you will never get a 5.56 mm chambered gun to work well or are your 3 suggestion independent and I can perhaps have 5.56 with a heavier spring and buffer? Also, I had always avoided 5.56 sbr’s because of the rounds coming out sideways- is this the case with these pistols or will changing the bullet weight give better stabilization?
    Thanks,
    Tim

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