So, it's looking like this might be the bonus season that I finally buy an M11/9 SMG. What should I be looking for in terms of buying one? I don't really know anything about them, other than I'd need a new upper to make it anything besides a bullet hose. (One thing that attracted me to the M11/9 was the number of nice uppers available for it: MAX-31, MAX-11, MAX-11/22, AA SABRE, etc.)