Now that's a new one for me. Had to think about it for a while. The only thing I can think of is the shell stops are installed wrong and somehow the action bar is interfering with them pushing the elevator up a shade?? If it were just the failure to fire and the cleaning happened I'd bet the neighbor botched the reassembly somehow and the disconnector is binding, it's spring is not attached correctly or something is keeping it from clearing. But the forearm??? has to be the shell stops, and the action bars.. Be sure to check the elevator pivot as the spring there acn be installed incorrectly,,but that really shouldn't involve the forearm being tightened!! arrrrghhh!! Now i'm gonna have to pull one outta the vault and look to see what else could do that! KEEP US INFORMED!!PLEASE!
Just thought about this some more.. Could the locking bolt cam or the carrier be improperly installed and the barrel when tightened into the receiver by the forearm be causing the carrier/bolt assembly to keep the disconnector out of battery.. Check function of the trigger with the barrel installed and the bolt locked with the forearm removed.