This gets weirder.
I've still not found the problem.
Usually, you rack the bolt, and either it won't lock, or it'll sort of 'half lock'. This is when the bolt is locked back, but if you eject the mag it'll slam forward again. You can see, if you look at the bolt lock, that it's not fully engaged.
Weird thing is, though, if you push the mag up into the magwell while you rack the bolt, it seems to lock back fine. You can eject the magazine, and do whatever, and the bolt will stay locked back.
Do you think it's the mag, or the bolt lock?
Ben.
EDIT:: Taken some pictures to