AR BCG function question

[nicaburns]

built an AR this weekend for a friend of my so based on an Anderson multi cal lower and an Anderson parts kit he purchased.
Uses an AIM surplus nib bcg and it would fire but not lock back after last round... Then it would not reliably pick up the next round. So I thought it might be the gas system.
It functioned fine with a different bcg... Could it be a gas key issue?

 

[user 71]

I would think gas key first. Does it seem loose or have any movement (from the carrier)?

 

[Cobalt 60]

If it is short stroking, which is what it sounds like, could be a number of things. Try an different BCG first. If that works, you know where your problem is. Bad gas ring fit can cause that as well - either through bad rings or bad gas chamber (not the correct term I don't think, but can recall the correct term) in which the gas rings reside. I have successfully polished out the gas chamber on a BC that was headed for the junk pile. Check the rings. pull the bolt out. stand the BCG on end, bolt face down. Push down on the carrier, and the bolt should retract smoothly and easily. If not, that COULD be the problem. However, that is not a definitive test. MT Dawg and a couple others more knowledgable than I should be along soon with some suggestions.

 

[nicaburns]

I put a different bcg in and it ran like a champ... I did not check the key for movement but we will hopefully be sending the bcg back to aim for a replacement since it started this right out of the gate... I don't even have a dozen builds under my belt so still learning

 

[Cobalt 60]

I put a different bcg in and it ran like a champ... I did not check the key for movement but we will hopefully be sending the bcg back to aim for a replacement since it started this right out of the gate... I don't even have a dozen builds under my belt so still learning

Oh, yeah - if it is new ship that bitch back!

I have also seen a peened gas key come loose, which could definitely cause short stroking. Damndest thing.

 

[Willy Leadwell]

Gas key doesn't have to be loose to leak. you can check it for leaks by putting some soapy water around it and putting compressed air into the key tube. I actually had one problem that turned out to be a leaky gas key and one that was mounted slightly crooked. Both yielded similar results, but the leak was much trickier to identify. Once I knew the key was leaking, I just put some 1000 grit sand paper on a flat surface and sanded the key flat against it. Then I put the sandpaper on the key and sanded the carrier surface. A little red loctite on the surfaces for a good seal and she ran like a champ. The crooked has key was a spikes BCG. I just loosened it, straightened it up and re-staked it.

 

[Wallace's]

If the gas key was still secure, I would say that possibly the chrome lining inside the carrier could have been out of spec causing increased friction with the bolt (along the lines of what Cobalt was saying). Did the BCG travel freely in the upper and engage the lugs easily by hand?

 

[Wallace's]

Gas key doesn't have to be loose to leak. you can check it for leaks by putting some soapy water around it and putting compressed air into the key tube. I actually had one problem that turned out to be a leaky gas key and one that was mounted slightly crooked. Both yielded similar results, but the leak was much trickier to identify. Once I knew the key was leaking, I just put some 1000 grit sand paper on a flat surface and samded the key flat against it. Then I put the sandpaper on the key and sanded the carrier surface. A little red loctite on the surfaces for a good seal and she ran like a champ. The crooked has key was a spikes BCG. I just loosened it, straightened it up and re-staked it.

And that didn't deform to gas tube when it cycled?

 

[nicaburns]

Did the BCG travel freely in the upper and engage the lugs easily by hand?

Yes... We will send this bcg back but I still want to learn from the experience

 

[Wallace's]

Yes... We will send this bcg back but I still want to learn from the experience

Cool, snap a few pics of the bolt and carrier separate before you send it. Wonder if anything looks abnormal.