OK, I have been talking up how much I like the Hornady 55gr Practice ammo, which is steel cased but very good quality. As a result, a friend bought a box of it to try with his AR. The new box had lacquered steel cases that look like the old Wolf; but were marked HMC and shot like 55gr Practice ammo (very nicely).

Every time he tried the ammo in his Armalite midlength, it would vigorously eject the fired round and then fail to load the next round (bolt would drive right over the top of the round without picking it up at all). However, using BH 75gr, Remington UMC, and PMC 55gr, the rifle functioned perfectly. Further this rifle has fired thousands of rounds of ammo with no function problems. It is extremely reliable.

My first thought was it was a magazine issue; but it repeated itself with known good magazines and the same magazines that did not work with Hornady Practice worked fine with other brands of ammo.

My second thought was "bad lot of ammo or funky new lacquer" so I gave him some of the earlier zinc-plated steel cases from my stash. Same exact result. In addition, I tried the lacquer coated in my rifle and a nearby Bushmaster; both of them ate it like candy with no problems.

So why does this specific rifle extract and eject; but fail to strip off and load a new round only with this particular brand of ammo (and using different lots even)?

Ejection of the case was right into the same pile as ammo that worked in the rifle and was consistent. Based on that and my observation of the bolt cycling, it didn't look to me like short-stroking was the issue. Fouling wise, the rifle had less than 200 rounds on it and has run with thousands of rounds between cleaning previously.