I purchased a .300 blackout 14.5 inch upper from a well known person. It has a Montana barrel, Bushmaster receiver and BCG. I started with hand loads but they would not cycle. It will fire but then short stroke. I purchased the recommended buffer and spring that AAC describe along with factory ammo. It still does not cycle. Suppressed or unsuppressed. It is a DI setup. Any ideas as what to try next? I thought maybe the gas port on the barrel is not aligned or the hole is incorrect diameter. Does anyone have an educated idea?
What’s the gas system length? And is there any possibility that the gas block is not aligned with the gas port? Who assembled it?
I had a similar issue,and I also thought my gas block wasn’t aligned, but it turns out two things were the issue: I needed to switch to a Carbine buffer, and use a BCG with the o-ring on the extractor spring.
I’m not sure of the exact length. But the barrel is 14.5. The guy who is assembled it is a former spec op man. He currently teaches sniping and other skills. I’m leaning toward alignment issues or barrel port size?
I have a carbine buffer and I’ll try the o-ring I didn’t do that yet. Thanks.
Have you asked him? Most people who assemble an upper will test fire it.
Gas system length should be fairly obvious as either pistol or carbine. Carbine on a 14.5" 300BLK is probably marginal.
I have two 16" 300BLK uppers with carbine gas, both run perfectly with supersonic loads (I haven’t tested any subs). I have not added an O-ring to the extractor on either one, although I didn’t check whether the BCG might have been assembled with one originally (LMT bolt in DSA upper, DD bolt in my own build).
It’s a carbine length. I also have a H2 buffer and a carbine buffer spring. I followed the AAC website recommendations for the upper. I am trying to get up with him he’s out of town till next week.
If carbine is marginal, what would you recommend?
You can start with normal troubleshooting.
Check the bolt and carrier for any binding.
Try a known good carrier out of an AR that functions well.
Use one round in the mag and check for bolt hold open.
I have a Ruger 5.56 I’ll try that BCG and check for binding. Thanks
Check the gas port size. I had the same issue with my 300 and had to open it up to .125" before it would cycle reliably. I have seen some ports measuring .180" which to me seems way too large but was evidently required in order to cycle. My gas tube was also misaligned in the gas block and had to be matched. Good luck!
Don’t think the Ruger is a good idea for test firing; its a piston system, not direct impingement, so carrier key is probably solid, not hollow, to allow gas into it.
Mark
Try an H buffer and/or carbine buffer. I actually dropped back to carbine buffers on two of my three BLK rifles.