So I had a planned plinking session with a few friends and was excited when my new AR upper arrived just as I was about to leave. So, like an idiot, I just slap it on my lower and go. When I go to shoot it, charging it feels a bit gritty, but it shoots a few magazines of Federal XM885 like a champ. Then I try some old (10 year+) assorted 55gr stuff I had lying around. It started short-cycling, usually the last 2-3 rounds of a mag. Changing mags didn't help and I had forgotten to bring some lube, so eventually I just fed it the 62gr. Because I just moved and my guns have yet to move with me (complicated situation having to do with an gun-fearing landlord who insists on a gun safe or cabinet), this is driving me nuts thinking about. My first lower was fine running dry with the same 55gr, and it was a Del Ton (relatively cheap). This new one was a PSA (usually regarded as high quality) so I figured it would do the same. I seem to be wrong. So my questions to all you more experienced guys are: Did I get a dud? Or are ARs just not good running dry and my first upper was a fluke? Or is my old crappy 55gr ammo to blame?