Well, I took off the cap, and it is the plug that is rattling. It fits pretty loosely in the hole. It seems like the cap should tighten against it though, but it apparently does not.
Interestingly, the lug on the barrel is mated against the base of the stud on the end of the magazine, and the barrel is locked in place. However, there is a 1 to 1.5 mm gap between the vent rib and the reciever. Is that gap normal? Just curious...
Thanks guys,
Wes