ms6852, I've personally found the Hornady practice match ammo to be very accurate, especially when compared to brass cased ammo of similar price. I'll guarantee you that manufacturers do not have mil spec ball ammo in mind when the make the sub MOA guarantee. They are talking about match ammo.
If you are looking for the utmost in accuracy from factory ammunition, Federal Gold Medal Match is a good place to start.
As for the steel cased ammo, if your rifle shoots it, then shoot it, if it won't, then don't. I suspect that much of the internet swag concerning steel cased ammo came about as a result of lots of guys not wanting to admit that their rifle wouldn't function because there is something wrong with their rifle.
If you're looking for a decent plinking round, check out the Silver Bear 62gr HP. I've had good luck with it and my plinking carbine likes it better than M193 or 855.
As for the comment regarding 7.62X39 doing better due to the taper of the case, I really don't know. My very first AR was a Colt chambered in 7.62X39 and I had all sorts of problems with cases sticking. Of course, if I knew then, what I know now, I may have been able to remedy the situation.
I think that steel cased ammo working better in AK type weapons has more to do with the chamber dimensions, dwell time, timing and cyclic rate. I've never looked up the specs or did any tests, but a full auto AR sure SEEMS to shoot a lot faster than a full auto AK.