Bolt wont lock back.

[Reelamin]

Need some help if possible. New build. My bolt wont lock back on last round, and will if I pull charging handle back on same empty magazine. I have only built with mid and carbine length AR gas tubes and carbine/pistol buffer tubes. I wanted to build a very accurate AR, and wanted a fixed full length stock. Aero barrel, Aero Adj Block, Aero rifle length tube, Mag Pul carbine tube/spring/buffer. My rifle length buffer tube will not fit the Mag Pul PRS Lite stock so had to go with carbine stock tube. I get good ejection 3-330 and at worst 5 with gas block fully open. Bolt will not lock back no matter what the gas block is set at. Take any info before I tear into it and start throwing changes into it. Easy to just put the rifle tube/spring/buffer on it, and if it works I need to find another stock that will fit it. I would prefer to just stick with the Carbine tube and my PRS Lite stock.
Edit....it is a Superlative Arms Adj. Gas block, Aero on the one that is running like a top. Put the rifle buffer tube, spring, buffer on and same results. Swapped out the Superlative to a standard Aero block and same results again.

 

[rpol98]

if we're going to make the assumption that you're not getting enough gas to drive the BCG back enough to engage the bolt catch, regardless of the gas block or gas block settings, then it's going to beg all the consequent questions about whether you have that gas port channel in the gas block directly above, and centered over the barrel gas port. I take a measurement of the distance of the gas port hole to the shoulder on the barrel, and also the distance of the gas port channel in the gas block, and do some math to get it just right. sometimes that ends up with the gas block forward of the barrel gas block shoulder maybe 0.020" or thereabouts, it varies, mfg variances, probably the thickness of the absent handguard cap. if you have a bore scope, sometimes you can verify that.

only after you've verified that, and that there's no obstructions in your gas tube, no obstructions in the BCG gas key, can you then consider whether the gas port hole in the barrel is sufficient. if you have a hole gauge set, that helps. in short, seems like something is either undersized, or restricting in your gas system.

you could also consider going to a slightly lighter buffer, or buying one of the gucci-brand springs that have varying spring strengths, a slightly weaker buffer spring. is it a standard 3oz buffer? or did you start off with a H2 or H3? I presume it's a standard AR-15 M4 carbine spring, not "extra power".

I'm sure I'm missing some other presumptions, or factors, others can chime in.

a friend of mine, dumb AF, bought a BCG that's optimized for suppressed use, for an unsuppressed gun. paid top dollar for that extra "feature". took a while to figure out that the BCG itself was restricting the gas, short stroking. but he's a bloomin' idiot. Hopefully you bought a normal BCG for your build.

 

[slikmrc]

One thing to check is BCG isn't contacting back of receiver .

 

[WanderingGoatie]

What magazines have you tried?

 

[rpol98]

you said that you're able to pull the charging handle all the way back, and when you do this, the bolt locks back. need to verify that it's the actual bolt face that's behind the bolt catch, because sometimes folks have the front edge of the bolt carrier instead caught behind the bolt catch, not the bolt face. In some unusual circumstances, it might be the bolt face caught up on the mag follower. anyways, something to verify.

in any event, need to verify that you're able to manually achieve full travel of the bolt carrier to the rear, bolt face behind the bolt catch.

 

[Reelamin]

Thanks everyone. What I get for wanting precision on a blaster design base. Latest update....I am a freaking idiot and no idea how I did it. When I installed the standard Aero I measured the gap from block to barrel shoulder and installed it. Verified with my dimple jig centering pin. Frustrated I took the gun totally apart and did not find any wear or marks. I pulled the Aero standard gas block and blew down it. I had the tube blocked based on what it sounded like and seemed restricted Pulled the gas tube re-installed it and tried again with the rifle buffer tube/spring/buffer wallah....runs like a top with proper gas. Switched back to the carbine gas tube/spring/buffer and again same as before 3-330 and wont lock it back. Put the Superlative back on after blowing through it and it seemed ok. Nope dang thing wont lock back and great gas with 4-5 turns out with the rifle and carbine buffer set ups. Opened it all the way up and same thing. I am out of gas tube pins so now waiting for some pins and will take it apart next. My aero adj. block worked perfectly on a mid length gas tube. Any way to tell how much flow I should be getting through the gas block to the tube?

 

[Reelamin]

What magazines have you tried?

Two cheap range, old and brand new mag pul, old GI. The spring pressure is good if I pull charging handle back, and with the standard aero block and rifle buffer tube.

 

[rpol98]

I re-use my gas tube roll pins, as long as there's sufficient force holding them in, needing a roll-pin punch and small hammer to get them in. of course, if it's a push-in slip fit, that goes into the trash can. Haven't encountered that yet. Bent, deformed, mushroomed, also go into the circular file.

but, that's your call, your choice.

 

[ARlife4me]

bolt traveling too fast will override the catch. very important question...........what cartridge!

 

[ARlife4me]

DITCH the SA gas block and buy a better 1. I've had 3 and all were bad. I have several AP barrels and components and all work great. I don't use or need an adjustable gb for any AP setup I have and that includes 3 6.5creed barrels without + length gas systems. All have carbine stocks with standard weight buffers and springs.

 

[UncleDano]

Thanks everyone. What I get for wanting precision on a blaster design base. Latest update....I am a freaking idiot and no idea how I did it. When I installed the standard Aero I measured the gap from block to barrel shoulder and installed it. Verified with my dimple jig centering pin. Frustrated I took the gun totally apart and did not find any wear or marks. I pulled the Aero standard gas block and blew down it. I had the tube blocked based on what it sounded like and seemed restricted Pulled the gas tube re-installed it and tried again with the rifle buffer tube/spring/buffer wallah....runs like a top with proper gas. Switched back to the carbine gas tube/spring/buffer and again same as before 3-330 and wont lock it back. Put the Superlative back on after blowing through it and it seemed ok. Nope dang thing wont lock back and great gas with 4-5 turns out with the rifle and carbine buffer set ups. Opened it all the way up and same thing. I am out of gas tube pins so now waiting for some pins and will take it apart next. My aero adj. block worked perfectly on a mid length gas tube. Any way to tell how much flow I should be getting through the gas block to the tube?

So, what you are saying is that you don't know if your barrel takes a carbine or mid-length gas tube?

 

[375ultramag]

Ar15 or Ar10? If ar10 my guess is ar15 length carbine buffer instead of ar10 carbine buffer. Just throwing my guess out there.

 

[ARlife4me]

Ar15 or Ar10? If ar10 my guess is ar15 length carbine buffer instead of ar10 carbine buffer. Just throwing my guess out there.

Yet we don't know which platform or cartridge.

 

[ARlife4me]

Ar15 or Ar10? If ar10 my guess is ar15 length carbine buffer instead of ar10 carbine buffer. Just throwing my guess out there.

If an AR-15 buffer was used in place of a 10 it could blow the back of the tube out depending on tube material. Happened to me by using a b5 buffer in the wrong place.