300BLK Subs at 9,300ft

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.

Here is the rifle BTW.
Attachment 57490

Lighter bolt carrier group and recoil spring?

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

Does it cycle at all? FTE or ??

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.

Quote:

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Originally Posted by 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.

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

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.

You really need to chrono your loads.

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.

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.

Elevation is a factory with speed of sound though. I want them to stay sub sonic.

I've chronoed, used NiB BCG's, reduced recoil spring, etc. They are subsonic and loaded pretty much to max without breaking the sound barrier. Even with a can, that should provide more back pressure, I'm surprised I can't get it to run. It ejects the brass but doesn't quite come back far enough to pick up another cartridge. I could load it slightly hotter and see what happens. If it starts breaking the sound barrier then I've got to go after the gas system. I agree with Tim and Mark. It's probably a gas issue. I'm pretty much dedicating this 8.5" upper to subs anyway so maybe I should put a standard block on it and enlarge the port slightly. I've heard it's real easy to make it too big though. May be calling you Tim.

Could you be overgassed? Too much chamber pressure with the can installed and the round is slow to eject.

My Sig716P would not cycle (full power ammo mind you) suppressed. The hotter they were, the worse it ran. The replacement model has been running well, but I'm not convinced the accuracy is as good.

I was just wondering the same thing. I'll bet he tried turning the gas down already, though.

I have a spare 8" barrel you can try if you want. The gas port is 0.125". I guarantee you'll have to turn the gas down...

Yes, I started with the gas low. I've literally shot like 100 test loads through this thing with different configurations of all kinds. It's getting expensive now.

I assume the bolt will lock back with the bolt catch manually. That proves that you probably have enough bolt travel.

Try another upper on the 300 lower. And try the 300 upper on another lower if you can.

Good Luck.

#31 drill for the gas port, and I've heard of reducing the buffer's weight to assist cycling.

Come get mine and swap uppers and lowers around till you figure out the problem.

Tim, is yours an 8.5" also? I've tried different lowers. I'll get in touch soon. I'm working on the East coast now.

Watch it be something cometely unrelated like wrong rear sight aperture...

It's 8 or 8.5", I forget exactly.

So.....my subs run great through my 16" barrel upper, even down to 10.6grn of AA1680. I think my next step is to replace my gas block with a standard one and/or tap the gas port out slightly larger. The pistol gas system vs the carbine system make a significant difference I guess.

My first question would be what weight buffer are you using? Is a lighter one possible? If so that is probably a cheaper and easier test that messing with the barrel- in my experience port sizes and buffer weights are pretty closely tied together (all that has been 5.56 exp but I can't see why it would be conceptually different). Second would be if you have tried something like the JP low mass BCG, potentially in conjunction with lighter buffer. Third would be do you plan to run supers as well in the future? If so, I personally would hold off on opening the port AND changing the gas block (again due to my experience with 5.56 and under/over gassing issues between combos of subs/supers/suppressed/unsuppressed). From what little I know of your barrel (grain of salt and all that) some people have gone out to .125 on it to get it to function well. I believe it's .096ish originally iirc, but anyways I'd start small on opening it up. My 2cents.

Very curious to see what results you have as my can should be out of jail in a month or so and I'll start shooting my 9" AAC 300 with subs in earnest.

I'm an idiot. I pulled the adjustable gas block off and was looking at it. I noticed the gas port was blocked with something that looked threaded. I thought the flush gas adjustment screw fell out and I had a replacement screw bought. Well.....the screw had worked it's way all the way in somehow instead. I was getting almost no gas and thought I was adjusting the gas with the new one. These are tiny screws. Long story short, I opened up the gas with the old screw, reset it, then shot it. My subs work fine in it so far. Live and learn. I shot 10.6 grn no problem and may go lower since they cycled fine.

Glad you got it sorted out.