Best AR pistol chambering

The .300 Blackout would be your best bet for short barrels, super- or sub-sonic, followed by a dead heat with 6.8 SPC and 7.62X39. With newer magazines and corrected feed ramps, 7.62X39 is now a viable option.

300Blk is your best bet, depending on your use other options vary after 300 blk. 7.62X39 is a good option because ammo is so cheap, but good mags are kinda few and far between. Available, yes. But nowhere near as available as ar mags. Pistol calibers work great in ar pistols as well, again depending on your use. Same problem with mags though as 7.62x39.

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astocks2622 said:

Yeah, I was looking at doing a pistol upper in 10mm as I already load that, but reliable mags & feeding seem iffy...

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This is my plan. My beef with 10mm is that most are still running blowback, and I want locked breech gas impingement setup on my gun. Mags are not an issue, run glock mags, buy or build a block to pin in place to locate the mags at the front of the factory magwell. It helps that I have a mill and do this stuff as a hobby.

300blk is not intended just for subsonic use. It makes an excellent SBR round in supersonic form. You are losing much less performance than you are in equally sized 5.56 SBRs.

In that regard it is very similar to 762x39. If you are looking at an AR platform, personally I would rule out 762x39 though. Last time I seriously looked at the possibility of getting one (years ago), my research basically indicated to me that they universally sucked from a reliability standpoint. Maybe they have been improved today? But even if they have, why bother with a bunch of proprietary components when 300blk will give you virtually identical performance and uses all the native components? I'd go 300blk all day.

Like any other firearm, what range and target is this intended to be used against? Express that distance in yards, specify if it's paper, dirt, or live, and then choose the ballistic application that fits.

It is literally that simple.

I chose 5.56 with a 10.5" barrel, same as what Colt designed in 1965 for the Army with the XM177, because it retains a flat trajectory well beyond 250 yards and carries 1,000 foot pounds out past 80m. I don't know of a pistol caliber round that does that. I use it for deer hunting in close cover during rifle and pistol season.

As for barrel length 10.5" is about the longest that works with standard carbine length buffers or pistol buffer tubes. A pistol is not over 26" or it loses that legal status in some jurisdictions. You could find a short buffer tube and buffer assembly but the price goes up exponentially when single source aftermarket parts are used - which is the issue with 6.8 or x39 - and it does let you use a longer barrel for the incremental gain in power. For the most part the alternative cartridges don't need it as much as they pick up more from their larger diameter sooner in the barrel. That means they work well down to 8" barrels if needed.

It still goes back to range and target - put things in numbers, like inches of penetration needed, and you then choose the cartridge that will work, along with it's optimal short barrel length.

If you want under 10" definitely go 300 blackout, if you're gonna go 10-11.5 any of them will work. I went 5.56 with a 10.5" and I'm gonna build a 8" ish 300 at some time.

Mavracer nailed it. The 300 Blk is fun to reload, tons of possibilities. Its close to a 7.62x39, but more versatile. I've got loads for 180 grain projectiles that will still be supersonic. I think of it as a 30 Carbine on crack.

300 BLK with pistol length gas system and an adjustable gas block.

I don't know how reliable they would be or if they are even made, but a short .458 Socom, .50 Beowulf, or .450 Bushmaster could be fun.

On the flipside, my 9mm AR is my favorite rifle. It's setup the same as my 5.56 gun, but more pleasurable to shoot in my opinion. Plus it will run in absolute filth without cleaning. I don't even want to know what it looks like in there right now.

I think of it as a 30 Carbine on crack.

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I think of it as a 357 mag carbine with pointy bullets lol

9mm or 7.62x39, or would be my recommendation less than 10", add 5.56 to the list if >10" barrel, the 7.5" 5.56 have too much muzzle blast and flash to be very practical, but they sure can be fun!

.300 blackout if you like/want to reload or don't mind shooting rather expensive ammo.

My 5" 9mm is my absolute favorite SBR, followed by my 10" 7.62x39.

Why build a pistol AR in a pistol caliber? What's the point of the world's largest an most awkward 9mm pistol?

You know what reliably takes 10mm Glock mags? Glocks.

Fun. That is the purpose. That is pretty much the purpose of any AR15 pistol (either that or waiting for a tax stamp). I'm not saying they aren't viable for some roles, but it isn't like the AR pistol is the gun of choice for clandestine operators.