I'm not so much concerned with lifespan in terms of number of rounds, but rather longevity with different types of ammo. I recall a test done by DPMS or Bushmaster (was on arfcom) where they put 10k rounds of 3 different ammo types through 3 different barrels... the steel case ammo showed a significant decrease in accuracy at the 4k mark. While I won't be shooting steel case primarily, I won't shy away from it if that's the cheapest thing on the shelf.

Mick, what are your thoughts on 11.5" vs 12.5"?