300BLK Subs at 9,300ft

[belizejet]

I'm sure I'm not the only one with problems but I cannot get my 300blk SBR to cycle at altitude. I live at 9,300ft. I'm using an 8.5" CMMG barrel with pistol gas system. I am also using a suppressor. I've loaded nos custom comp 220 grn with 10.6, 10.7, 10.8, 10.9, and 11 grn of AA1680. I also loaded some Hornady 225 grn with 10.8, and 10.9grn of Aa1680. They all stayed subsonic but none of them cycled. I even bought a reduced recoil spring and have tried both a M16 BCG and an AR15 BCG. Any suggestions? Everything on paper says if I go higher with powder loads then I'll break the sound barrier. The factory Remington 220grn subs won't cycle either. Full power supersonic loads work fine. Most of the data I have is from lower altitudes. I was also reading that temperature makes a huge difference also.

[belizejet]

Here is the rifle BTW.

[275RLTW]

Lighter bolt carrier group and recoil spring?

[brutal]

Lightweight BCG was my first thought. Slick it up too, NiB or one of the fancy coatings.

Does it cycle at all? FTE or ??

[Tim K]

Are you sure your gas block is lined up on the port? What size port is in the barrel? I had to drill a gas port larger to get one barrel to cycle.

[Great-Kazoo]

Are you sure your gas block is lined up on the port? What size port is in the barrel? I had to drill a gas port larger to get one barrel to cycle.

^^This^^ I'd check performance at lower altitudes also. Is it the load, or a firearm issue.

[MarkCO]

You need more gas. Check the gas rings, gas key alignment, gas block alignment. If everything is good, you might need to open the gas port. Altitude will have almost no effect on the cycling.

[20X11]

You really need to chrono your loads.

[Tim K]

Mark is right, elevation is irrelevant. You're talking about a change of 1 or 2 psi compared to the pressure in the gas system of thousands of PSI.

[belizejet]

Funny, I've done most of the above. I think I'm just going to change the gas block. It's a cheap double star adjustable and it's full open. When I get to look at it I'll check alignment. It's has always run supers just not subs.