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Old December 18, 2019, 02:40 AM   #1
scorpion_tyr
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7.5” Barrel AR 5.56 or .300 Blackout?

I have this crazy idea of building a cheap AR to keep in my vehicle. An AR pistol with a 10.5” barrel is just a little too long for where I want to keep it so I plan on building one with a 7.5” barrel.

The excuse is is that I need an AR to go with the 9mm and the SHTF bag I already keep in the vehicle. The real reason is that I just want to build another AR, but I don’t want the warden... oops, darn autocorrect... the wife to find another new gun in the gun safe during the weekly shakedown.

The plan so far is a PSA stealth lower and parts kit, Shockwave Blade, Magpul grip, and either a PSA or Bear Creek upper. The only question is should I stick with 5.56 to maintain ammo, magazine, and parts compatibility for SHTF or should I go with .300 AAC in case the short barrel doesn’t give the 5.56 enough velocity to really do its job?
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Old December 18, 2019, 11:57 AM   #2
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I have a 7.5" 5.56 and a 10.5" (or 9.5"?) .300 Blk.
The 5.56 is fun, but I don't ever, ever want to shoot it without hearing protection - and, good God, it would be punishing if ever fired in an enclosed space.
Velocity wise, it's fine. There's still enough there to be effective.

.300 Blk is not as bad on the flash and blast front, but still wouldn't be my choice if I was building another one or starting over. (Mine was built specially to be suppressed. I see little point in .300 Blk unsuppressed.)

If I was building another or starting over, it would be 9mm.
Cheaper ammo. Cheaper to reload. Plentiful defense ammo. And if suppressed, 147s are automatically subsonic.
I would opt for 9mm.
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Old December 18, 2019, 04:03 PM   #3
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I’d hate to have to fire a 7.5” 5.56 from inside a vehicle. I’d recommend you try firing an 11.5” or 16” 5.56 from inside a vehicle (preferably one you can afford to damage) before committing to that plan. I don’t even like 7.5” 5.56 outdoors.

So, I’d go .300 as it is noticeably milder with blast and flash; as well as better ballistics.

I don’t know what your budget looks like; but a LAW Stock Adapter will let you fold an AR15 stock for more compact storage. If you want to be able to shoot with the stock folded, there are also options like redesigned bolt carriers that have the springs entirely in the upper, or the SIG MCX uppers. In either case, that would give you a shorter package while keeping useful barrel length.

I’m not a huge fan of pistol caliber carbines since if I’m going to carry something the size of a carbine, I want it to hit like a long gun; but FrankenMauser makes some good points concerning the role you have in mind.
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Old December 18, 2019, 04:44 PM   #4
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6.5 Grendel...
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Old December 18, 2019, 05:15 PM   #5
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I have a few of both, 556 and 300blk SBR’s. I have in the past had a 7.5” 556. I will never own another. Just ridiculous in terms of blast and flash...horrible for a useful rifle.

The shortest i will go on a 556 is 10.5” and 12.5” is better.

My 300blk SBR is a 9.5” AAC gun, but it wears a Gemtech can 99.9% of the time.
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Old December 18, 2019, 05:33 PM   #6
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.223 out of a short barrel is crazy loud even with double hearing protection. While you do what you have to do to save you life, expect permanent hearing loss if you shoot it in an enclosed space. I'm not sure how much quieter .300 Blackout is, but it is supposed to be quieter. I was thinking of getting am AR pistol as well, but I really don't have much need for it.
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Old December 18, 2019, 10:46 PM   #7
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Keep the 10.5” and get a law folder or maybe pre order one of those buffer less BCG’s from Evolution Weapon Systems. Maybe find someone who has a 7.5” in 5.56 and see if how you like shooting it before you build it.


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Old December 18, 2019, 11:22 PM   #8
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Thanks everyone! I don’t mind loud bangs and bright flashes at the range. Hopefully I’ll never have to fire it from inside the car. In most of those scenarios The 9mm would be faster to get to anyways (and would still be loud).

I’m curious to see how much louder it is than my SU-16. That gun is so loud it’s almost funny. I’m definitely leaning towards the .300 AAC now. My 10.5” blackout is absolutely wonderful so I might order a 7.5” upper to at least try out.
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Old December 28, 2019, 10:11 PM   #9
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if SHTF you're more likely to find .223/5.56 ammo a lot easier than .300 BO
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Old December 28, 2019, 11:30 PM   #10
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Quote:
if SHTF you're more likely to find .223/5.56 ammo a lot easier than .300 BO
You may want to reacquaint yourself with forum rule 5.g.
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Old December 28, 2019, 11:31 PM   #11
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Originally Posted by FrankenMauser View Post
You may want to reacquaint yourself with forum rule 5.g.
I'm not following you.
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Old December 28, 2019, 11:32 PM   #12
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Quote:
Originally Posted by scorpion_tyr View Post
I have this crazy idea of building a cheap AR to keep in my vehicle. An AR pistol with a 10.5” barrel is just a little too long for where I want to keep it so I plan on building one with a 7.5” barrel.

The excuse is is that I need an AR to go with the 9mm and the SHTF bag I already keep in the vehicle. The real reason is that I just want to build another AR, but I don’t want the warden... oops, darn autocorrect... the wife to find another new gun in the gun safe during the weekly shakedown.

The plan so far is a PSA stealth lower and parts kit, Shockwave Blade, Magpul grip, and either a PSA or Bear Creek upper. The only question is should I stick with 5.56 to maintain ammo, magazine, and parts compatibility for SHTF or should I go with .300 AAC in case the short barrel doesn’t give the 5.56 enough velocity to really do its job?
my response to this?
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Old December 29, 2019, 01:37 AM   #13
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Old December 29, 2019, 01:43 AM   #14
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Sorry for using the abbreviation. What I had in mind to be better prepared for is a scenario such as the Zanesville massacre. A scenario where I may need to share ammunition with others unexpectedly. Unlikely, sure... but possible.

And the real reason is just because I want to build another AR.
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Old December 29, 2019, 07:40 AM   #15
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I built an AR pistol in 300 BO and put an 7.5" barrel on it for a number of reasons. First, it's compact and easily carries/stows in tight places. My state is a constitutional carry state and has a capacity restriction for semi-autos of 5 cartridges while hunting for anything with a barrel over 8", so I put a 7.5" barrel on. With a conventional flash hider it's pretty loud--but not unreasonably so--meaning I have lots of other guns that are just as loud or louder and I have no problems firing it for extended sessions (with hearing protection). Inside 75 yards the 300BO is going to be very bad news for whatever it hits with some of my supersonic handloads.

I'm also a huge fan of 9mm AR's, though I haven't built anything with a barrel under 12." They are very easy to make, the very definition of limitless cheap ammo available. The most fun you can have with an AR for plinking IMO is a 9mm. In your case, that would also have the advantage of being the same cartridge as your handgun.

The difference between the two--IMO--is the over-all reliability of the 300 BO is superior to the 9mm, the 300 BO fires rifle bullets and the 9mm fires pistol bullets which tend to be a bit more "sticky" on chambering depending upon the bullet profile.
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Old January 1, 2020, 09:10 PM   #16
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I have AR uppers in ,22lr, .223 Wylde, and 6.5 Grendel. The one round I will never have an upper for is 300 BO. This is because I personally find the risks of the BO to exceed any possible benefit in my case. The vast majority of kabooms in ARs in recent years involve a BO round getting fired in a 5.56/.223 rifle. I don't like that the design of the BO allows one to be chambered in the smaller bores. The Grendel won't do that, so I don't lose sleep over a possible KB. YMMV.
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Old January 1, 2020, 10:45 PM   #17
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Quote:
I have AR uppers in ,22lr, .223 Wylde, and 6.5 Grendel. The one round I will never have an upper for is 300 BO. This is because I personally find the risks of the BO to exceed any possible benefit in my case. The vast majority of kabooms in ARs in recent years involve a BO round getting fired in a 5.56/.223 rifle. I don't like that the design of the BO allows one to be chambered in the smaller bores. The Grendel won't do that, so I don't lose sleep over a possible KB. YMMV.
I only own wood-burning, steam-powered vehicles for this very reason.
The risk of fuel system and engine damage are just too great for diesel and gasoline cars, when accidentally filled up with the wrong fuel.
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Old January 2, 2020, 12:30 AM   #18
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I shoot a lot of customers' guns, and the most unpleasant to shoot are the shorty ARs in 5.56. In an enclosed space, it is hard on your sinuses and head due to the pressure/concussion. I opted for and would recommend a 9mm carbine, but given the options you stated I would go with the 300 BO.
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Old January 2, 2020, 12:28 PM   #19
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The vast majority of kabooms in ARs in recent years involve a BO round getting fired in a 5.56/.223 rifle.
I’ve found that by paying attention to what I’m doing, and putting the correct ammo in the firearm eliminates this issue.
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Old January 2, 2020, 12:47 PM   #20
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Properly made 300blk ammo will not chamber in a 223 barrel.

I put a heavy crimp in all my 300blk loads. The bullet should run into the shoulder area of the chamber and prevent the bolt from closing.

If the ammo is too short this can be a problem.
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