300 Blackout Pistol with CMMG 8.5" barrel

[JoeHarley13]

Here's my latest creation that I built about 2 weeks ago. I took it to the range last Saturday and fired it 40 times for the first time. It worked great! No jams or malfunctions of any kind. I shot 110gr Hornady SP, Remington CTFB 115gr, 125gr Sierra SP and mostly 147gr military pulled surplus. Only 4 rounds were factory ammo, can't find that stuff anywhere.
The 147gr zip along at 1,850 fps. This generates 1120 ft-lbs. which is more energy than I get out of my 223 with a 16" barrel shooting Hornady 55gr VMax. Only the NATO 5.56 rounds & a few HOT 223 rounds out of a 16" barrel barely beat my 8.5" AR Pistol. So, I'm pretty happy about the performance of the little pistol.

Specs:
Upper Receiver: Spikes upper receiver
Lower Receiver: Spikes lower receiver
Barrel: CMMG 300 AAC Blackout 1:7 8.5" Nitride barrel, Pistol gas
Bolt Carrier Group: RGuns Nickel Boron coated
Gas tube: Pistol length gas tube
Gas block: Syrac click adjustable low-profile gas block (.750), uses (2) set screws
Muzzle Device: PSA A2 flash hider
Lower Parts Kit: PSA Lower parts
Trigger: PSA Mil-spec trigger
Grip: Magpul MOE, FDE
Rail: Midwest Industries SS Gen 2, Carbine length
Stock: PSA M4 Carbine
Buffer Tube: Generic pistol w/foam pad (from LGS)
Buffer: Generic pistol (from LGS)
Charging Handle: Vltor BCM Medium
Trigger Guard: Magpul MOE trigger guard
Pins: standard now, but plan to upgrade to Gen 2 KNS Anti-Rotating Pins when I upgrade the trigger
Sights: Magpul MBUS
Optic: Primary Arms red dot
Magazines: Magpul Gen 2, FDE; Lancer L5 translucent. Both have had the ribs filed down slightly

[WHA1949]

Reason why you run your front sight backwards?

[JoeHarley13]

just ignorance I suppose. Sighted in nicely at 25 yards, guess I need to correct and verify
It was keeping them all inside a 1.5" circle, 5 shots touching was my best at 25 yards.
Opened up considerably at 50 yards with the iron sights & even the red dot. probably 4" group..
No plans to scope it.

[eastern_hunter]

What gas tube did you use?

Might you please measure the exposure of the tube into the inside of the upper for me? I built an upper with that bbl and it is short stroking. Either the gas tube is a bit short, or the port is not big enough.

Thank you!

[JoeHarley13]

I'll measure it tonight.
I used this DPMS Pistol length tube from MidwayUSA. http://www.midwayusa.com/product/288594 ... ngth-ar-15

Only have my adjustable gas block open 5 clicks (1.25 turns) out of a possible 16-18 clicks to run Supers. No short stroking.

[JoeHarley13]

Measures .930", measured along the bottom of the tube.

I have a .040" gap between gas block and the shoulder on the barrel for proper gas block/hole alignment. Sometimes people try mounting it flush, but in most cases it isn't.

Bummer you are having problems, Good luck!

[eastern_hunter]

Thank you VERY much for the information! Is very valuable to minimize the possible places for error.

Intention is to use this upper with an Octane can for subs only. Am running a standard carbine buffer and spring. Using an M16 carrier and Stag bolt. Barrel has a TROS 3 lug adapter on it so that it is just longer than a Specter length YHM light rail. The interior exposure of my tube is right at the same as yours, and the carbon mark around the gas port indicates the YHM gas block is properly centered.

Could be ammo ... have been trying to develop load with 225 gr Hornady OTMs using 2400. Using more powder than the ammo that does cycle in my 16" pistol gassed Paladin bbl. Really need to chrono it though.

I suppose it could need some break-in. Might have to open up the gas port.