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something easy to check , as it happened to me on a pistol
measure the inside depth of your buffer tube
I think you want 7-1/4 inches deep
if you have another rifle handy check that one and see what it measures too
the one I had on the pistol did not lock back the bolt empty mag, buffer showed signs of dings on the plastic endcap which I believe was caused by hitting the inside of the tube as it wasnt long enough
the buffer tube was 1/4 inch shorter then 2 other ones I checked
once replaced locks back the bolt as it travels back a touch more to allow this
but at that same time I had put in the next heavier buffer but im pretty sure it was the tube length
now if I had left it maybe shooting a thousand rounds would of deformed the plastic cap on the buffer enough to lock back but as it was the buffer was some china brand and started to unscrew im sure it would of just broke apart so thats why I replaced it at the same time putting on new tube
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Something tells me that the issue involves the words "gunsmith" "Thunderball gas block" and "pinning in place".
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To those of you who mentioned that the bolt catch might be the problem, I say, 'well done'. Once I opened the receivers and looked at the different magazines interacting with the bolt catch that came from San Tan, I could see that the arm on the front was too short to let the mag follower reliably push the bolt catch up.
Then I left town for many weeks and didn't work on it. But now there is a new bolt catch in the lower and it has no problem with being lifted by any magazine.
Thanks for the responses.
Bart Noir