I've got a DPMS A-15 that has never caused me any problems, and I shoot Wolf and surplus mostly, some Rem green box and Win white box, whatever is cheapest for practice. Some of the surplus I have is pretty good, like the Radway and LC stuff, so I practice with the commercial.
I also have a DPMS A-15 lower I am building - or will be once I get unpacked from the move - into a 6.8. The Stag upper for 6.8 was 575, assempled 6.8s are going in the $1k-1400 range