FTE with bolt lock-back?

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My gun has an 11.5" BCM 5.56 barrel, A5 system, and SLR adjustable gas block. BCG is a DD with standard 5-coil spring and black insert.

When tuning the gas to run well with the suppressor installed, I begin having issues with the can removed (duh). Most are typical short-stroking symptoms. One is consistent and usual enough that I thought I'd mention it.

With either H1 or H3 A5 buffers, when I dial the gas back to the point where operation becomes an issue unsuppressed with weak ammo (PMC Bronze), I get bolt lock-back and FTE.

That is, I stick an empty mag in the gun, drop a round in the chamber, send the bolt home. Fire. The casing will occasionally eject to 4:00, but 4/5 times its left sitting on top of the mag follower, with the bolt locked back. This happens with several types of ammo, and is a very reliable failure mode that happens a click or two before the gas is so weak that I don't get bolt hold-back.

Of course the same thing happens with a full mag, only then it's a double-feed or stovepipe that results after the bolt comes home.

Since ejection is strong and reliable when normally gassed (zero failures of any kind in about 500 suppressed rounds with many kinds of ammo), I don't think there's any issues with the extractor or ejector. Both feel good to me.

This is sort of a non-issue: I don't expect the gun to run unsuppressed when the gas is choked way down. I was just not expecting to see consistent FTEs AND reliable bolt lock-back. In the past, I've always seen lock-back go first, while the gun is still running fine otherwise. So, I'm curious what, if anything, this means. Is it unique to the A5 setup? It's my first gun using it.

[Iraqgunz]

I would have advised to get rid of the adjustable gas block. I would also omit the O-ring.

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How would you set up a suppressed SBR without adjustable gas? Just let it run way overgassed all the time?

There's no o-ring in the DD BCG as far as I know. I haven't taken it apart to find out.

[MistWolf]

You have an extraction problem.

Chances are, you are not getting double feeds. A double feed is two live rounds jamming the action. From your description, you're getting a spent case and a live round stuck in there. That is indicative of a failure to extract.

Before doing anything else, fix your extraction problem

[Slvr Surfr]

Definitely remove your BCG and check and clean the extractor. Also see if you have a Crane O-ring installed. With the bolt out, check your ejector and make sure its able to move under good pressure. If it's spongy, the spring may need to be replaced.

I had similar problems running my 10.5" sbr. I needed to use the black rubber O-ring to get additional pressure on the extractor rim shooting suppressed. DD makes great stuff, but I feel that running a suppressed SBR is different animal. In order for my bolt to keep a hold of the case rim I needed to use the black O-ring on the extractor spring.

My rifle is also using the Syrac AGB and A5 system. As Grant has mentioned in another thread, tune your rifle to the point that it will extract the last round properly, but not lock back on empty when un-suppressed. That way it will still be smooth when running suppressed and also lock back due to the added back pressure.

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Thanks guys! I was thinking gas and blinded to the extraction issue.

I put in a Bushy BCG that was running well, and was able to turn it down until the bolt didn't lock back, but I got extraction. Subjectively, the DD extractor felt softer. I put in an extra-power spring I had around, and the DD BCG worked great from that point on.

Right now unsuppressed, with gas 3 clicks out full-power ammo cycles but doesn't lock the bolt back, and the ejection is weak. My low-power ammo doesn't cycle.

At 4 clicks out, my .223 pressure ammo has unreliable holdback, but cycles well and ejects strongly.

4-5 clicks feels about right with the can on and full-power ammo, so this is right where I want to be!

[Slippers]

How would you set up a suppressed SBR without adjustable gas? Just let it run way overgassed all the time?

Basically, yes. Although it's not that overgassed, really. For example, I run an 11.5" bcm and a5 extension, and normal gas block. Shoots fine suppressed and unsuppressed.

The ejection pattern shifts forward a little, but that's it. There's no tuning, and it shoots my range reloads or full power 5.56mm without any problems.

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Wolff extra power spring on the left, DD on the right. DD came with the insert that is shown in the Woff Spring.



Standard spring from a rebuild kit of unknown make on the left, DD on the right.