A couple questions about a .300 AAC Blackout build

[Snowdog]

I've been wanting a .300 BLK pistol upper for a while now and decided to go with PSA's 7" MOE Marauder that's selling for $289.

https://palmettostatearmory.com/psa...der-ar-15-upper-assembly-black-no-bcg-ch.html


This upper does not come with a charging handle nor BCG, so I intend to remove those from a S&W Sport II that I don't use (it's pink) and slap those in this 7" upper.

Some buddies have made mention that I really should use a bolt carrier group (and magazines) made specifically for the .300 BLK, but I don't think that really matters. Am I wrong?

Can't I simply use the BCG from another .223/5.56 AR and use that for this build? The Magpul gen2/3 magazines should work too, right?

Thanks in advance!

 

[JeeperCreeper]

I could be wrong, but I always thought 300 BLK was designed to use standard 5.56 BCG and mags.

I just assumed anything labeled specifically 300blk was marketing

 

[MistWolf]

There 300 BLK & 5.56 both use the same bolt. Life is easier if you have a BCG for each upper.

The 300 works better from 300 dedicated magazines.

 

[troy fairweather]

Ya same bolt and mags, it's easier to get new stuff and Mark it so it's clear there for the blkout.

 

[edwardware]

Some buddies have made mention that I really should use a bolt carrier group (and magazines) made specifically for the .300 BLK, but I don't think that really matters.

Your buddy is ignorant, and perhaps also stinging from being conned himself. The bolt and BCG are identical for 5.56/.223 and .300BO.

Mags are interesting. 5.56PMags will restrain your COAL more than nonPMags, which will restrain your COAL more than .300BO-specific mags. It's still trivial to setup COAL to work in 5.56PMags; I do without issue.

The trick is to set COAL so that the .308 bullet presents the same diameter as a 5.56 neck, at the internal rib in the mag intended to restrain the 5.56 shoulder. If it's too long, the rib cocks the cartridges centerward in the mag, instead of them laying straight. PMags have a very pronounced rib, others less so, and .300BO-specific mags have none.

I load .300BO, and don't own a single .300BO-specific mag.

The solution, for long/heavy subsonic bullets is .300BO-specific mags, but not for light supers. You'll run out of neck before you run out of magazine. The solution for supers is 7.62x40WT, or maybe .300HAM'R, which will utilize the entire mag length with 110-150gr bullets.

 

[Demi-human]

I like physically and visually different magazines for the Blackout, to keep them from going in a .223 AR.

Everything else but the barrel is the same.
Your bolt will work, but it’s better to have a BCG for each rifle because they mate to each other. One bolt, one barrel. It’s better, not that it won’t work.

I have had Blackouts feed from all my magazines fine. Even the bushmaster feeds from most of them. Just don’t feed them to the wrong rifle!

 

[DoubleMag]

Can't I simply use the BCG from another .223/5.56 AR and use that for this build? Yes. Caveat below. The Magpul gen2/3 magazines should work too, right? Again, yes with caveat


The caveat is, you should absolutely gauge the bolt to the barrel. If you have a buddy with 300BO gauges, you're in. Otherwise it's a rental or purchase. Buy is around $70-80 for both Go and NoGo. 2nd cav. e. on mags, if you are going to shoot the heavier bullets such as 190s, 208s go with the ones labelled for 300BO. Otherwise don't worry about it and I wouldn't worry even with a regular mag.

 

[Cannibul]

I'd look for a 10.5" upper.

My first 300 Blk was a 7.5". Obnoxious little beast. I built a 10.5" and it's much more pleasant to shoot, more accurate, and just balances better.

 

[Gordon]

7" really starts to degrade the performance of supersonic ammo.unless you are going suppressed I'd go 10" or longer. No less than 8"+. You will be using pistol length gass under 10", they like heavy buffers.