Go to Remington's website and download the manual. There is a drill bit that will clean the gas ports out without increasing their size, I don't remember what size it is but the manual will tell you. Also, make sure the O-ring is good and that the metal ring next to the O-ring is not installed...
If the deal sounds too good to be true there is a catch. Night vision, NBC gear, lights, anything that you don't usually see for sale anywhere else is suspect. A price that seems really low could indicate the seller doesn't have much equity in the item. (Nice way to say it is hot.)
I took four up there last year to get checked out. Got all four rebuilt and a new frame for one that was cracked. No charge. They treat you like they appreciate your business. Just one of the reasons I will be a Glocker for life.