[John in AR]

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. It’s 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 I’ve 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 I’d 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 o’clock 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 I’m glad I took it slightly larger than standard.

 

[John in AR]

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.

 

[Kilibreaux]

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.

 

[Kilibreaux]

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.

 

[John in AR]

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...

 

[John in AR]

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.

 

[BigBassMan]

You mean you are not using a dremel and a tap, pounding screenwire, making your own!

 

[John in AR]

The almost-final iteration, with Mystic-X in place:

I took that pic in september and just forgot about it. Still have the cmc trigger and seekins short-throw safety to install; just been crazy busy with real life and haven't taken the time yet. Most of the shooting I've done lately has been with .22, and not even a ton of that.

 

[Garand]

interesting project!

 

[John in AR]

An example of AR’s being like a Mr. Potatohead for grownups, still tinkering to get this gun exactly how I like. I know I still want to mount a light (that I already have on hand), and a b.a.d. lever (that I’ll have to order), but other than that, think I have it in what should be its final state.

If it continues to be as reliable as it now is, it may replace the 16” 556 gun as my ‘primary’. Never expected to say that, but having the power of my long-used 16” 556, and the compactness of an sbr-size gun while avoiding the shortcomings inherent in an sbr-size 556-caliber gun, the only real downside is losing the cheapness of 556 ammo.

I’ve settled on lancer magazines for .300bk, nhmtg magazines for 223/556, and have sold or given away all my other AR mags except several Elanders, that I may or may not hang onto.

 

[Dorobuta]

The almost-final iteration, with Mystic-X in place:

I took that pic in september and just forgot about it. Still have the cmc trigger and seekins short-throw safety to install; just been crazy busy with real life and haven't taken the time yet. Most of the shooting I've done lately has been with .22, and not even a ton of that.

I have the same holosun and brace on my .300 pistol. I have a 10.5” barrel with Mlock instead of keymod.

 

[John in AR]

Only thing I didn't like about the Holosun was the battery tray. Never had an actual problem with it, but it seemed flimsy and I was paranoid about the tiny screws getting lost or stripped. It worked fine, but I found a great deal on a used MRO on ebay (when the new generation came out), so I swapped it out. The narrow 'blade' brace was originally on an old-style 9mm blowback ar carbine, and was surprisingly uncomfortable after just a few magazines. So I put it on the blackout, and eventually moved it to the .22LR where recoil isn't an issue. My new upper is also a 10" with Mlok instead of keymod, and very happy with it.

One thing I changed when I changed uppers was that I now leave a Mystic-X fixed-barrel adapter on the gun even when the suppressor's not on it; as in the pic from yesterday above. When the suppressor isn't on the gun, it serves very much like a kak "flash can" blast diverter, helping direct blast away from the shooter.

It does make the OAL an inch & a half or so longer (~9.5" instead of 8") than with the low-profile mount in the Mlok upper picture, but almost have to have something out there when the suppressor's off, so may as well be that piece that serves a purpose both with and without the suppressor body itself. If I just 'had' to make the gun as compact as possible, I could go back to the low-profile suppressor mount, but the extra 1.5" or so doesn't really bother me; it's only a 10" barrel to begin with, so still not much longer oal than a 16" gun with a flash hider.

 

[Dorobuta]

I agree on the battery tray, but for now, it hasn't been a priority to change it for something else. At some point I'll buy some scope or RDS that catches my eye and I'll do the whole musical scopes thing where I have to go sight in several rifles because I swapped optics on all of them. I guess those are good problems to have.

 

[John in AR]

Whenever Aimpoint, Trijicon, etc, introduce a new "latest & greatest" optic, all the "gotta have the latest & greatest" guys buy the new one, so there ends up being deals on used previous-generation versions on ebay just a few weeks later. I have four MRO's, and they've all been bought this way; a WHOLE lot cheaper than new ones.

 

[John in AR]

Also fwiw, I learned a lesson on this gun from a problem I'd never encountered before, and I've been shooting AR's for a little over 40 years. The last round in the magazine would always jam, ending up with the tip up higher than it should; missing the chamber opening and smacking into the area above the chamber. It would very rarely do it at other times as well, but it ALWAYS did it on the last round in the magazine. This was all with full-power supersonics; Fiocchi 150 fmj and Remington 130 Hog Hammer.

I tried it with new L5AWM magazines, almost-new NHMTG/Okay, and almost-new Elander gen-3's. The gun had a CMMG 8.5" upper at the time, and the only modification I'd made to it was a Superior Arms click-adjustable gas block. To test bolt speed, I tried the gas port nearly closed, wide open, and several settings in between, with no change in last-round jamming. The ejection pattern varied between 1 o'clock and 5 o'clock, depending on gas block setting. The bolt locked open when empty, so it wasn’t short-stroking. This is what it looked like, every time on the last round of the magazine:

I posted the question in a PM to a user here, and also posted it publicly on the AR15.com 'troubleshooting' section. Everyone at arfcom pretty much universally suggested trying a heavier buffer, so I gave it a quick try one night. The buffer in this gun was a normal 2.96 oz carbine buffer, so I pulled an H2 buffer out of another gun and tried it. That helped immensely, and evidently the core of the problem was that its forward speed was too fast, and the reduced spring pressure of the last round in the mag meant that the bolt moved forward too fast, before the round was completely up & ready for it.

After looking into things further, I decided I needed a solution that slowed down the forward speed without slowing down the rearward speed. After a bit of contemplating, I went to an even heavier H3 buffer, and a reduced-power buffer spring. On the rearward stroke, the added mass of the buffer offsets the reduced spring pressure, and on the forward stroke, the heavier buffer slows things down just enough to make the difference. Works like a champ, but never had to mess with any of that before on any gun with a rifle-length or mid-length gas system. Only this one pistol-length gas system & SBR-length barrel needed that tweaking.

 

[John in AR]

Hard to believe it's been five years ago that I started this build and this thread, and recently got the gun set up in what I believe will be its final state. Basically, finally got around to setting it up so it mimicks my main gun, which is a 16" in .223 caliber.

The two biggest things on it for me personally are the seekins 60-degree ambi safety and the "b.a.d." lever. You wouldn't think that an aftermarket safety lever would make much difference, but it makes a lot to me; letting my trigger-finger knuckle engage the safety as I take my finger off the trigger. It took a little bit to get used to it, but it's really intuitive and automatic now. When my finger goes into the trigger guard, the thumb takes the safety off as is normal, but when I pull my finger out of the trigger guard, the knuckle now knows to ride the side of the gun and the safety is 'on' as soon as my finger is out of the guard. Really like it.

On the b.a.d. lever, I use it as much as for locking the bolt back as for an actual bolt release; for clearing the gun, dealing with a malfunction, whatever. It also took some getting used to, but been using them for 8-10 years now and am a big fan.

 

[BigEd]

John, What you said in the threads about buffers and springs for this round is giving me pause on what I wanted to do.
I was going to sell one complete 5.56 Pistol and use the funds to build a nice complete upper for my other 5.56 Pistol. Now I'm seeing a possible issue so I'm thinking of just selling the upper and matching up with the lower.
Or I may put the whole thing on the back burner until after I move 😑

 

[John in AR]

...buffers and springs for this round...

From the reading I did at the time, it’s not an issue with this round in particular, but an issue with pistol-length AR gas systems in general. The only SBR-length AR I’ve ever had in the past was a 10.5 inch 5.56 with a welded 5.5 inch flash hider on it, back nearly 30 years ago. It was unacceptably twitchy and unreliable, but I just always assumed it was a fluke of that particular gun. I didn’t know at the time that it’s actually a common issue with AR’s with short barrels and short gas systems.

In combination with the heavy H3 buffer, I ended up using a “yellow” buffer spring from Sprinco.com. Their stuff came highly recommended in a lot of places, and definitely worked out well in my gun.

Sprinco USA -- Recoil Solutions

 

[BigEd]

Understand that.
In the past I had similar troubles with ona number of carbines and one early AR pistols back in the day.

My two current builds have run fine. However I'll make an educated guess that me using the complete SBA3 brace has had something to do with that. As I assume that they have a buffer and spring designed to work okay with an average 10.5" bbl and gas port. I could be wrong and just have gotten lucky the last two times around.
I'm mainly trying to figure out the route of "less hassle" and time.😁