Did you clean the 870 before you fired it? Mine came filled with some sort of nasty grease that was pretty stickey. Well. filled may be a bad choice, but there was quite a bit of residue on the bolt and inside the chamber. In typical do as a say, not as I did fashion, I took mine right from the box to the range. I fired 10 or 15 shots, just to see how it felt, then took it home and cleaned it. It did seem to smooth out after I cleaned it. Mine is the standard 870 Express Magnum, not the Marine, but I would think Rem would treat both the same at the factory as far as packing grease.
------------------
CCW for Ohio action site.
http://www.ofcc.net