Winchester 5.56 that I bought in December. First round fired from from my mini 14 was underpowered. Sounded like the pop of an M 16, little recoil, more than a .22 but not much. The action cycled and the next round loaded, but it sure didn't feel like I thought it was going to. Looked like a hit low at 6:00. That was actually a fold in the target, it didn't hit paper at 25 yds. Since I thought I saw a hit, I fired the next round. Ba- boom. Much louder, MUCH more recoil. I checked the barrel - no "knot" or bulge, so I figured out the first round was bad. Fired 40 rounds and all were like the second one. I returned the package to my local gun store with the offending round brass. They marked the lot number, etc. guess I better check the rest of the Winchester ammo I bought from them.
I was lucky. Had it not cleared the barrel when I thought I saw a good hit on the target I could have had a burst barrel. Since it was the first 5.56 I fired since a familiarization firing on the M16 in 1972 I did not realize it was a "short round".
Oh, mine was FMJ 55gr.