CMMG 300 Black Out [Archive] - Glock Talk


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11-24-2011, 12:10
I asked CMMG to build a 9" 300 Blackout upper for me. I thought it would be a perfect size for an entry weapon and home defense using a suppressor and sub sonic rounds.
I was totally unfamiliar with the 300 Black out round but read up on the ballistics and limitations.
For me, it would be a short range weapon, though it looks good to 300 yards.
Ammunition has a wide range from 124 gr. standard ammo to 220 gr. sub sonic.
The carbine is set up with a pistol length gas system for the 9" barrel and subs. The supplied gas port was .089. That is the standard for a 16" barrel with a carbine length gas system so after consulting with a friend of mine who has done a lot of testing with these, I decided to open the gas port to .110. I thought if it was over gassed, I would add a JP adjustable gas block. As it ended up, I won't need to.
The carbine:
9" pistol gas length CMMG barrelled upper
Midwest Industries 9" T-Series Free Float Hand guard
Midwest Industries Low Pro Folding BUIS
Primary Arms Micro Dot
UTG 3X Magnifier with integrated folding QD mount
SAS .30 Caliber Suppressor
Lower is a Bushmaster
Kies MK1 FCG
H2 buffer/carbine spring
Ammunition was CMMG 147 gr. Standard Velocity - 2000 FPS
CMMG 175 Gr. Subsonic -950 FPS
I was not shooting for groups, but function. The carbine functioned perfectly with standard 147 gr. ammo as well as 175 gr. subsonic ammo unsuppressed.
No signs of being over gassed in respect to markings on the brass.
The standard ammo ejected about 4 feet at 4:00
The subs ejected about 4 ft. at 3:00
With the suppressor:
147 gr. Standard Ejected about 4-5 ft. 3:00-4:00
175 gr. Sub Sonic Ejected 4 ft. at 3:00 (I had some rounds eject 1-2 ft. at 3:00 More on that later)
While I was not shooting for accuracy, I was surprised at how accurate and consistent these rounds were. I guess I expected something erratic from a heavy sub sonic round. Not the case. I had groups about 1" at 50 yards with open sights and the Micro Dot.
The CMMG 175 gr. subs hit 1" lower than the 147 gr. standard velocity. Acceptable for my purposes.
The PA Micro Dot worked well, was plenty bright on 3 for the cloudy day. Dot is round, sharp and clear. Nice and compact clone of the Aimpoint T 1. It's a great little optic for the price. I do recommend getting the ADI QD base up grade. I have one on the way. The PA mount works fine but PA told me if I remove and remount the optic much, the ADI is the better mount and has better return to zero.
The UTG 3 X magnifier is clear, the focus made the image nice and sharp. The simple side flip mount works well. You just pull back and flip it to the side.
Down side of the UTG magnifier is it's on the heavy side compared to some higher end magnifiers. But I'm pretty happy with it.
The Midwest Industries 9" T-Series Hand Guard was perfect for this application. It left enough room for the flash hider/suppressor mount to clear the blast. The T-Series has an inside diameter wide enough for the SAS .30 caliber suppressor to fit inside. The suppressors OD is 1.5"
I've used the M.I. low pro BUIS in the past. The work well. Easy deployment, locks in place when deployed.
The front sight has an HK type hood with standard Mil. elevation adjustable sight post. Folded, the profile is very low and out of the red dot's field of view.
The rear sight is windage adjustable with positive clicks and a dual sight plane aperture. It works well with a scope or magnifier as it folds very flat and leaves plenty clearance for the optic.
One issue I had. A friend sent some 1000 FPS sub sonic 175 gr. rounds, but I don't think they were loaded to that. They rounds were very quiet. They function and cycled 90%. They were too underpowered for the H2 buffer. Ejected about 1-2 feet. A couple times the round chambered but the hammer did not reset.
The rounds on the target were 4" lower than the CMMG standard velocity 147 gr. and the CMMG 175 gr. Subs.
Shooting subsonic rounds with the suppressor was comfortable without hearing protection. Even under a canopy.
There is a difference in loads and POI. A big difference.
Choose what load you will run, dial it in and stick with that load.
I was very impressed with the reliability, accuracy, compact size, sound reduction in a manageable package.
The upper with suppressor still measures 16 1/2" barrel/suppressor length. Meaning, it's the same length as a 16" carbine without a flash suppressor.
The 300 Blackout uses the same 5.56 bolt and magazines. All it takes is a barrel change to give you a .30 caliber with ammo choices from 124 gr. to 220 gr.
And let me tell you. The round hits with a thump!
Dave N

11-24-2011, 12:25
Nice stick and write up!

I've been happy with all my CMMG and M.I Purchases....I'm curious about the UTG which model is that? Most of the models I see online are listed for Airsoft/Paintball applications.

11-25-2011, 17:08
I'm not sure it's on the market yet. They send me gear to test.
UTG has upped the game in the last year, since moving the operation to the U.S. The only negative I found with it was the weight. It's a little on the heavy side but seems well built.
It's not an Aimpoint or Eotech magnifier, but then it is just a magnifier. I like the built in side flip mount. Very simple and works well. Locks in both positions.
Dave N

11-29-2011, 09:35
Awesome build, that's a pretty stick!

Can you give any personal experience based comments contrasting the 300 BLK to the 6.8 SPC? I'm thinking about acquiring an upper in one of these calibers in the coming year. Thanks.

11-29-2011, 10:38
If you're using Primary Arms and UTG on a serious rifle.......

I'm not even going to bother at this point.

Have fun with your new toy, but when stuff breaks, you were warned. Hope it doesn't cost you your life.

11-29-2011, 14:05
I hate to sound like a snob,......but the PA & UTG optics have no place
on anything more "hard use" than a .22LR.

11-30-2011, 08:18
O.P., are you a Navy SEAL kicking in weathered wooden doors in the Hindu Kush after fast-roping into the area?

Are you a LEO SWAT?

If not, and that rifle is for nothing more than poops and funzies, disregard the above posters opinoins about your optics and rail choices. They'll work just fine if you're not abusing the hell out of your rifle. If you take care of your rifle and it's for basic civilian use, you'll be fine.

11-30-2011, 09:47
I have a couple firearms w/ PA micros and some w/ Aimpoints, I realize the difference b/w the two brands. They have different limitations and each have their very distinct AOs.

FAA and I have PAs on our .22s but not on our other rifles. We speak from experience.

11-30-2011, 12:17
If you're using Primary Arms and UTG on a serious rifle.......

I'm not even going to bother at this point.

Have fun with your new toy, but when stuff breaks, you were warned. Hope it doesn't cost you your life.

I agree.. get that crap off your otherwise nice rifle.

11-30-2011, 13:44
Yea pa stays on range guns but if I'm gonna depend on it for anything besides that I want something heavy duty