7" BCM 300 Build Issues

Moderators: gds, bakerjw, renegade

Post Reply
Team 9x19
New Member
Posts: 3
Joined: Sat Feb 18, 2023 5:57 pm

7" BCM 300 Build Issues

Post by Team 9x19 »

Hey all,

Curious to get your feedback as I've run into feed/function issues, which I believe are due to BCM under gassing this 7" upper.
Build is as follows:
BCM 7" complete 300bo upper
Radian Lower with Geissele SSA trigger
Carbine length buffer Spring
Various Buffer Weights from Carbine, H, H1, H2, and H3

The gun cycles supers fine; however, it will not cycle subs. It will extract 190 gr Hornady Sub X, but the bolt will not come back far enough to load a round from the magazine, and on single rounds will not fully lock back. This problem is exacerbated with heavier Hornady 208 gr AMAX loads where I've had to use the Carbine buffer weight - otherwise I'm getting FTE/stove pipes.

1. I have not yet had time to disassemble and verify the gas block is aligned, and that there aren't any leaks. The BC should have a crane o ring installed, and I will verify that as well.

2. Provided there aren't leaks or anything else to chase it leads me to think that BCM simply drills the gas port too small, and focuses on reliable function with supers. If opening the port is necessary to reliably cycle subs and supers, does anyone have a recommendation on starting size? I do have SA AGBs on hand to use if necessary, but try to keep these types of parts used to a minimum if possible for "reliability's sake". BCM would not tell me the gas port size as it's "proprietary information."


Thanks in advance.

- Team 9x19
Tugnut
Member
Posts: 24
Joined: Mon Apr 25, 2005 11:41 am
Location: Tucson

Re: 7" BCM 300 Build Issues

Post by Tugnut »

The first thing I'd do is take off the gas block and measure the gas port. I'd think you'd want it to be at around .09" or slightly larger.

You could also mess with a lighter BCG and/or Sprinco reduced power buffer spring.

Are you shooting it with a suppressor?
Team 9x19
New Member
Posts: 3
Joined: Sat Feb 18, 2023 5:57 pm

Re: 7" BCM 300 Build Issues

Post by Team 9x19 »

Sounds like a good plan - re: measuring gas port. I have shot it both unsuppressed and suppressed, and across the board the results are virtually the same. I should give Sprinco a call and see what he recommends. I presume a yellow.. I also thought about cutting a coil (at a time) off the current carbine spring and going from there, but still want to address the cause of the symptom/issue, vs bandaiding it.
Tugnut
Member
Posts: 24
Joined: Mon Apr 25, 2005 11:41 am
Location: Tucson

Re: 7" BCM 300 Build Issues

Post by Tugnut »

I have a Faxon Match Gunner 7.5" with a 1 in 5 twist. I don't know the port size (gas block was already installed), but I thought I read somewhere it was .12"?

Anyway, my gun was short stroking with the weaker subs and ejecting empties but not feeding the next round unless I used a sub with a lot of gas volume. So I used a semi auto BCG I had laying around (the lightened bcgs were more than I wanted to spend at the moment), the lightest carbine buffer I had, and the Sprinco YELLOW spring like you mentioned.

My gun cycles much better now. I get reliable cycling now with subs that lots of handloaders say won't cycle an AR because the powder is too fast burning and there isn't enough gas to work the action. BUT, I don't get bolt hold open 100% of the time (that's not all that important to me). If you're using factory subs and a suppressor, I really think doing the same tricks I did will get you functioning 100%. No way my gun runs with my subs without a suppressor.
Team 9x19
New Member
Posts: 3
Joined: Sat Feb 18, 2023 5:57 pm

Re: 7" BCM 300 Build Issues

Post by Team 9x19 »

Good info - thanks for sharing! I'll call Sprinco and get a yellow on order. Ultimately I don't mind drilling out the gas port a bit. I rather have an over gassed gun for reliability's sake. It's weird.. I'm on the fence about it based on the benefits of running a good super/rifle round like a 120 gr vs a heavier sub for ballistic performance. I'll keep this thread updated as I tweak the gun.
Tugnut
Member
Posts: 24
Joined: Mon Apr 25, 2005 11:41 am
Location: Tucson

Re: 7" BCM 300 Build Issues

Post by Tugnut »

You're welcome.

Don't forget to let us know what your gas port size was when you take off the gas block.

I built my gun around subs, but if I do decide to get into supersonic loads, I'll just throw on an adjustable gas block like the Aero Precision one that Brownells has for $50. I know it's kind of lame, but I always try to pinch pennies when it comes to ammo and buying components (widowed dad trying to raise two kids). So what would push me over the edge to go the super/sub route with this AR is if I ran into a major honey pot of once fired 300blk brass at the range that no one wanted to bother picking up. All my current brass is devoted to sub loads, so if I were to come into 200 or so pieces of once fired brass, I'd use those for supers exclusively and get that gas block.
FartinSpartan
Member
Posts: 36
Joined: Thu Feb 23, 2023 3:52 pm

Re: 7" BCM 300 Build Issues

Post by FartinSpartan »

Don't feel bad I've been hearing a lot of that "proprietary information" lately too.

The funny thing is if their stuff was so great people wouldn't be having problems with it. Why would I want to copy/knockoff something (else) that doesn't work right?
Post Reply

Who is online

Users browsing this forum: No registered users and 62 guests