Yeah I should have been more detailed. All the Mossbergs I've ever bought seem to all come with a bunch of grease/dried grease in them and outside of them. 1st off like any weapon, clean it before you bother taking a shot out of it and especially shotguns.
When I got it I cleaned a lot of gunk off the outside and inside. I had a box of clays (needed to shoot something) and a box of 100 Federal 2 3/4 7.5 1200fps shot $20 Walmart cheapies. (The SX version is definately not a skeet gun)
It didnt like(at 1st)a few high speed "hip" shots meaning the butt was on nothing just to my side. Overall it cycled fine with the Walmart cheap stuff and after the 30th shot or so, it hasn't had a single problem cycle regardless how I've held it
I keep 2 3/4 00 Buck in it and have put a box of those through it without issue. I did fire a few BB 3" loads but those are my $$ Geese loads and didn't want to waste them playing around. The Model I have is 7+1 security model but i'm not sure if thats 7+1 with 3"....Never tried it.
My buddy has one and his didnt load that great until he cleaned it. He said it was filthy, but it is fine now. I don't know what he was using as far as ammo.