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I have an AK74 that worked like a champ for over 500 rounds but then the bolt suddenly stopped locking on a round. With no round in, everything cycles smoothly -- the bolt rotates into locked position without any trouble. Once there is a round in the chamber, though, the bolt refuses to rotate the final quarter inch or so into full lock (and the bolt is a pain to pull back out).

Here is what I think I have ruled out:

--The gun is put together correctly
--The barrel is not blocked (rounds go in cleanly by hand)
--I am not riding the handle down
--There are no apparent burrs on the rails
--Lubrication is not an issue
--The bolt seems to be OK and is clean
--The piston moves freely
--Ammo or magazine issues

Any suggestions on what else I should look for?

Thanks!