300BLK SBR Subsonic Cycling Issues

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300BLK SBR Subsonic Cycling Issues

Post by sunchaser73 »

I could use some help from the hive.
I have a 10.5" SBR that is experiencing issues running suppressed subsonic rounds. Here is the background info:
IMR-4227 powder @ 10 grains
Hornady 190 grain Sub-X projectile
Roscoe 10.5" barrel with a (modified) 0.125" gas port
Dead Air Nomad-30 suppressor
Direct Impingement gas system, non-adjustable gas block

When I first completed this build, I ran into cyclic issues. I progressively opened up the gas port and then converted to a piston system (Adams Arms) and a 0.125" gas port diameter. When I arrived at that configuration, I was able to get the gun to run subs reliably. Then, it started to malfunction. Ultimately, I arrived at the conclusion that the gun was over-gassed. While running a Sprinco Yellow spring and a 3x aluminum weight buffer, I could achieve last round lock back, but rounds would not feed reliably. After converting back to DI, I had a short run of reliable cycling, and then the issues cropped backed up.

I've worked my way up to the heaviest possible buffer (3x Tungsten weights) to no avail. I've tried an adjustable bolt carrier group (Bootleg BCG) in DI configuration and a heavier buffer spring (standard carbine).

Point being, I've been all over the map and I keep arriving back at the same place: an unreliable rifle that is intended to be a home defense weapon.

At this point, I'm looking at the following options to get the rifle to run:
1) Retry the adjustable BCG with heavier buffers
2) Go back to the piston system and try heavier buffers
3) Dial back the powder charge in an attempt to reduce the gas pressure
4) Try an adjustable gas block (I've had great success running the Superlative Arms adjustable GB on other platforms)

If you're still reading this post, I'm open to whatever suggestions you have to get this SBR running reliably.

Silent But Deadly
Posts: 394
Joined: Tue Apr 15, 2014 2:09 pm

Re: 300BLK SBR Subsonic Cycling Issues

Post by popper »

You only have 9k psi at the (pistol?) port so dia won't make much difference. Nor will heavy buffer/springs. A2400 will give more pressure, 11k psi.
Seems like when the BCG gets dirty, cycling stops for you. My numbers are for no can so your pressure will be a bit higher. I use a 190 cast with 2400 or CFE pistol, works fine but I don't shoot subs, @1400 fps.
Silent Operator
Posts: 54
Joined: Fri Sep 30, 2022 7:53 am

Re: 300BLK SBR Subsonic Cycling Issues

Post by Augenblick »

Pistol length port?

I don't have any experience with IMR4227 but have loaded 190gr sub-x with H110, A1680, and CFE BLK.

I have loaded these for Sig rattler 5.5" piston and a PSA 16" with pistol length DI. These are easy 300 BLK powders to get almost anything to run.

I don't have experience with using piston conversion kit and how that affects function. 0.125" would be more than enough for a DI system and you would probably want an AGB to back that down especially running suppressed.

What type of cyclic errors are you having?

A guy on another thread was having FTFeed issues with 190 gr sub-x that were resolved loading COAL 2.15".

Most people can get anything to run with A1680 or CFEBLK. Not the quietest powders but if you gun doesn't run gotta start somewhere.

You said you had subs running reliably at some point. Were those 190 gr sub-x also?
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