I am having a very minor issue with one of my ARs. When I get to the very last round in the mag the bolt will hold open. I am pretty sure the issue is narrowed down to a specific magazine but I want to ask and see if there is anything else to check.

If it is a mag what could the problem be? Possibly a weak spring?

Anyway the mag is a 20 round PMAG... I am pretty sure I have it narrowed down to which magazine. Let me know if I need to do anything to fix that mag.