.300AAC pistol build - Arms Locker
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Old 02-17-2017, 02:51 PM   #1
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.300AAC pistol build

Never have messed with AR pistols before and recently did something new, gradually piecing together a .300 blackout pistol over the last few weeks; basically my birthday present to myself. Its likely to stay mostly a play-thing and possible hunting gun, but I still very much wanted it to be reliable. AR pistols tend to be twitchy about reliability to begin with, and the larger bore diameter and widely varying power levels contribute to giving a .300bk pistol issues of its own.

Fwiw on the gas port, while most reviews & tests Ive read call for a 0.125 port on a .300 blackout for optimum reliability worst-case (unsuppressed subsonics), this CMMG upper had only a 0.079 gas port on it. Combination of smallish port, short 8.5 barrel, and .30-caliber bore diameter made it initially reliable with only a very few supersonic loads, and no subsonic loads.

Finally broke down and modified the gas system some. Picked up a superior arms adjustable gas block, carefully enlarged the gas port in the barrel, and now is all good with most loads, whether subsonic or supersonic. Only caveat is that even with the adjustable block and substantially-enlarged gas port, it still will not cycle completely with a small number of subsonic loads when unsuppressed. Neither the Remington UMC 220 OTFB or the Gorilla 208 Amax will fully cycle, whereas the Remington Premier 220 OTM and the Armscor 208 Amax reliably will.

What surprised me was that without any adjustment to the gas block, typical supersonics will also cycle just fine. I assumed that the setting that subsonics liked would be too much gas for supersonics, and figured Id have to turn it down some when going from subs to supers to avoid over-gassing; but surprisingly it does just fine, with the empties of both landing in pretty much the same pile right at 4 oclock or so.

I opened up the gas port gradually to 0.132, wanting to give the adjustable gas block as wide a range as possible. Even with that large a port, the gas block still has to be nearly full-open to function with unsuppressed subsonics, so Im glad I took it slightly larger than standard.
 
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Old 02-17-2017, 02:59 PM   #2
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In addition to being my first foray into adjustable gas blocks, this is also my first experimentation with Holosun red dots. This is the 503CU with solar backup. I like the concept, but don't know enough about them to trust them yet; maybe some time on this gun will help see how good they are (or aren't).

Beyond that, it's pretty standard stuff so far. PSA pistol lower, CMMG 8.5" upper, KAK buffer tube & flash can, slightly-extended BCM charging handle and magpul backup sights. I've picked up another seekins short-throw ambi safety and CMC trigger same as my primary carbines have, but haven't put them on yet.



I've only put a couple hundred rounds through it at this point, but am pretty happy with it so far. Even with the unmagnified optic, with a cheek-weld on the buffer tube it'll drop the armscor 208 subsonic rounds on a 12" rock at 160 yards almost every time; and I suspect that the rounds that do miss are my fault, not the gun's.
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Old 04-17-2017, 01:21 AM   #3
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EGGCELLENT build!

It LOOKS like you have a Krink brake on the nose, which is designed to make an AK-74 "pistol" length cycle and will do the same for you if set up correctly.

I use the exact same setup to make my 8" CMMG barrel cycle with 208 gr. A-max subsonics as it absolutely will NOT cycle reliably without the power boost from the muzzle device!

The Krink has a reverse cone inside that if you screw it all the way down doesn't have quite enough opening to admit enough gas to increase backpressure. So the answer is to grind off about .25" from the little "spout" sticking out of the cone then screw the entire brake down fully. You'll find it will then reliably cycle subsonic loads.
 
 
Old 04-17-2017, 01:23 AM   #4
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BTW, when you shoot supersonics you will want to remove the internal "cone" and just have the open Krink to REDUCE backpressure that is not needed...the cone is ONLY to improve cycling with low-pressure subsonics.
 
Old 04-18-2017, 01:07 PM   #5
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It's actually a KAK flashcan, not a krink brake. I've also since added a shockwave pistol brace. With the click-adjustable gas system, there's a 'sweet spot' of settings where it does fine with all the supersonics I've tried and most subsonics as well, as set up now. With my existing small 9mm suppressor (abraxas), it runs all but the UMC subsonic stuff fine.

Being a subcompact 9mm suppressor, I don't put supersonic .300's through it. Once the latest suppressor (a mystic-x) gets out of atf jail, I'll check subs & supers side-by-side and see how things are then.

We're at around day 200 or 205 now waiting on the suppressor to be released. It's already at my local dealer where I can go fondle it; just waiting on atf paperwork...
 
Old 04-18-2017, 02:20 PM   #6
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Took a pic of it as currently set up. Even with the short 8.5" barrel, it launches 110-grain bullets over 2,100fps.


Not a major fan of keymod, but it's what came on the upper and doesn't really bother me. So far am liking the Holosun. Have been leaving it on in battery mode, with both reticule elements lit up to see how battery life is. Been "on" constantly since putting it on the gun; only been a couple months now but so far, so good.

Will take another when the Mystic-X is on hand; am very much looking forward to that. It's nearly a pound lighter than my old M4-2000 suppressor.
 
Old 04-18-2017, 02:51 PM   #7
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You mean you are not using a dremel and a tap, pounding screenwire, making your own!
 
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