300 Black out or 7.62

Morning everybody!

So Im trying to start another AR15 pistol build. I already built a 9mm 6in and a 556 10.5in. Im trying to go for a 300 black out now. I would like a nice and short 4-5in, but theres one thing worrying me, the burn rate of the 300blk.

When I was doing research about building my 556 I was told that under 12in barrel length the 556 wouldnt burn all of the powder appropriately and a ball of fire would burst out. I went ahead and built a 10.5 anyways. They people I spoke to recommended either 7.62 or 300blk out since they would be able to fully burn before the bullet left the barrel.

What would you all recommend in barrel length and caliber? Im currently looking at 300blk in a 7.5, 8, or 8.5 in barrel. I would like even shorter, but I have not found an upper in that size yet.

 

Other considerations building a 300B. Are you using a can? Mass of the bullet and are you intending to run sonic or subsonic. I know people who ran into cycling issues. An adjustable gas block helped.

Just to cloud the issue the 6.8 was designed to run with short barrels.

7.62 I guess you are talking about a AR10? Never tried to build one.

 

I have no idea. Which one is more common (price/availability wise); subsonic or sonic? I planned on running it with the only accessory being a red dot.

 

300blk (7.62x35) will give you a lot more ammo and reload options, and better accuracy. 7.62x39 is cheaper.
7.62x39 was designed for a ~16" barrel (Russian AK), so you should have better burn rates with 300blk especially with heavier bullets.
Even a standard AK throws a dissent fireball, especially with commercial ammo. Short barrel will be bigger, but the muzzle device will make a noticeable difference too.
Also, 7.62x39 AR mags have always been finicky because the mag needs very curved and the rounds have a hard time keeping the proper angle going up the vertical AR mag well.
I'm assuming that you aren't talking about about 7.62x51/308 in AR-10. That's a big round and a whole other discussion if you want to shoot it from a short barrel.

 

I had no idea 300blk was also a 7.62 lol...yeah I guess Ill have to go with 300blk (7.62x35). It may be a little pricier, but then again I aint gonna shoot it much. Thats what my 9mm is for. Thank you

 

I found my next project @ALI3N_123

 

Badass bro

 

300blk is a cut and re-necked 5.56 case with a 30cal (.308/7.62) bullet seated. Some brass is modified 5.56, some is new dedicated 300blk. Russian 7.62x39 is a 0.311" bullet.
Look up iraqvet8888 on youtube, he has a video on different 30 cals and another on 22 cals. Goes though all the different variations.

 

I built a 300 Blackout pistol with an 8.5" barrel - no suppressor, standard buffer. I've run about 300 subsonic rounds through it without any cycling issues.

 

Great to hear bro, thanks

 

If you’re not going to suppress it do 8” 300blk. You’ll want to run supers unless it’s just a range toy then subs will be fine. I’ve built 2, 7.5” and 8” and used subs and supers suppressed. Neat round for sure. My latest project was a 10.5” 308, too damn rowdy. If going 7.62x39 I’d go the MPAP route before building off the ar platform.

 

Also, KAK makes something of you want super short and are willing to do some tweaks. I used their barrel for my 10.5” AR10 and it works almost flawlessly.
https://www.kakindustry.com/ar-15-p...0-blk/4-75-inch/300-blackout-4-75-inch-1-7-5r

 

Thats pretty cool, but I feel its going to take a lot more than what I want to put into it. Im building a collection of plinkers for range use, and then once I figure out what I really want, what I really really want, I will consider investing more into a dedicated self defense weapon. Im thinking for home defense I'd still prefer a shotgun.

 

Man you have more self control than me.... I say “this will be a budget build” every damn time. I said that when I went to build my ar10 pistol and Ended up with this:

 

I mean my budget builds LOOK like that, but one is all PSA one is half PSA half Delta TT lol. So one last question...for now...can I fire both sub and super sonic out of a 300blk upper like this one: https://palmettostatearmory.com/psa...de-9-lightweight-m-lok-upper-with-bcg-ch.html. ??

 

@Wixo As far as barrel length, if you are +/-0.5" then it won't make much difference. In that length range I think it would be dictated buy who's barrel you want and how much it costs.

 

Have you shot that across a chrono? I remember doing some calculations for a project like that, and bellow 12" or so the ballistics where the same between 300blk and 308 because of powder burn rates.

 

Hey OP,

I was recently in the same boat, deciding on length for a .300 blkout. Ultimately went with DDM47P @ 10.5" with plans for running a can (waiting for the ATF to hand'r over). One point I came across when getting some input was, 'dwell time'. I believe that refers to the amount of time/distance the gas keeps the BCG pressurized before leaving the barrel. I didn't dive too deeply into it, but it's something you may want to consider.

Another TW member's build (awesome setup IMO), @Patriot_Taco, is what got me started down that path. I think his is a 7.5 barrel.

His build: https://www.tacomaworld.com/threads/nw-wa-meet-bs-thread.224544/page-2295#post-19575195

 

I’d you ever come to Austin C&C you’re welcome to take mine for a spin, 8.5” barrel. Put in the tax stamp for a can couple months ago then I’ll be switching to subsonic. Till then just running whatever’s cheapest since it’s not the most wallet round ever

 

Yeah I wanted to be sub $1,200 with the whole gun and ended up spending $800 on the receiver set and hand guard. On that upper you’re good with sub/super. I used psa for both, the latter had a fucked barrel from them and they refused to warranty it. For your use, they will work fine. PSA is the cheapest I’d go.