I hope this is the right place to post this but I'm looking for some help troubleshooting a bolt lock back issue. The rifle was originally a 16" Bushmaster .223/5.56 with a front sight post and I believe a 1:9 twist barrel and standard Carbine stock. I have shot at least 3000k rounds through this so it is definitely broken in and have had virtually no malfunctions for the 8 years I have had it.

I recently built a SBR upper for it for a 300BLK build using an 8" CMMG barrel and it seems to cycle just fine both suppressed and unsuppressed. The only problem I have is that on the final round unsuppressed it does not lock the bolt back, but it's fine suppressed. I'm 99% sure this is a gas issue.

Weird part of this story is that I tried this same suppressor combo on my old reliable bushmaster upper, and now I have the same issue - no lock back on final round unsuppressed, fine when suppressed. This is with the same Gen 2 PMAGS I've had for years with no problems at all. The only thing that has changed when shooting unsuppressed is that I am not using the standard AR-15 compensator and have switched it to a flash hider that enables me to mount the suppressor.

The rifle is not overly dirty, and nothing has changed on the original upper except the muzzle device, which was not a brake to begin with so I do not know why this would have any effect. I've checked the bolt rings and they are fine and in tact. I suspect that maybe my spring is starting to go -- does this happen?

My next step was going to be changing out the spring with a JP Silent Captured Spring because I wanted one anyway, maybe I should get the spring kit to try the lighter spring options? Thanks for the input guys.

Of course, first post and I messed it up, I meant to put this in the troubleshooting forum... Apologies mods...

Try different mags
Try different buffer weights
Different spring
Check gas rings
Properly lubed

Topic Moved

Already tried different mags, plus they worked fine suppressed, doesn't that mean they're fine and the bolt carrier just isn't moving back far enough?

Not sure what buffer comes with a bushmaster carbine, but do they make lighter ones to counter the low gas problem? As I said I plan on getting the JP SCS so I can play with the springs on that.

Gas rings are fine

It could probably use some lube as I run mine pretty dry, but never had a problem before...

I guess I just find it weird that I all of a sudden have a gas issue on my unsuppressed 223 upper which was pretty much flawless until I took off the standard compensator and added a trifecta flash hider. I agree it's a gas issue though but does this make sense?

I'll definately give the silent captured spring from JP a shot, probably with the optional spring kit. I'm thinking standard JP buffer (vs the heavy tungsten) and probably start with a heavier spring and work my way down until I find the lightest one that allows it to lock back unsuppressed. Anyone see a reason to give the heavier tungsten option a chance?

Here's a link to the kit I'm talking about. Notice there's an option to change the buffer weights to tungsten ones.

bump... does anyone have experience with this?

Thanks for the reply, very great info!

Is my thought process incorrect that a heavier buffer would reduce the amount of backwards travel of the bolt which seems to be my problem? Or is the additional weight being propelled by the gas key enabling the buffer to build up more momentum to push it all the way back? If so, would this mean that I need a heavy buffer with a lighter spring? Thanks again for the help

I'm fairly certain I've figured out that it is an under gassed issue, and the bolt is not cycling back specifically unsuppressed. It cycles just fine suppressed with no out of the ordinary recoil. Do you still think the buffer is the place to start instead of the spring?
I'd prefer not to buy multiple weight sets for the JP Silent Captured Spring as well as the springs, as we're talking 200ish dollars for the setup and additional weights when I think what I'm taking away from this is that a heavier buffer would affect my setup negatively. Thanks again for your inputs!

It's running the standard carbine buffer from bushmaster, and I was shooting 69gn black hills ammo. The only change I made was changing the muzzle device from a standard ar15 compensator to a trifecta flashider/silencerco mount. After this it stopped locking back on the final round. Suppressed it looks like it may be throwing the case to the 2 o'clock position which may be an overgassed problem. I imagine this is just the nature of going suppressed to unsuppressed, just want to find the happy medium

TTT

Well, I swapped it out with a bone stock JP silent buffer and it locks back every time now, suppressed and unsuppressed, 300blk upper and 223... It's nice when something simple fixes things

The JP system is the best I've ever used, I put them in every rifle now.  The only thing I've had to do is add a quarter (or washer) in one lower to get the right bolt catch spacing.  The 9mm version really purrs on full auto.  I've been planning on getting a spring kit to do some bench testing.