If you get mud in the action, piston or no, it won't run. ARs tend to be a lot tighter tolerance than AKs, which is where the reliability comes from in the AK platform. If you hose it down with lubricant and don't get it covered in mud and dirt, and AR shouldn't have problems either way. A clean weapon is a happy weapon, a dirty, carbon filled rifle is going to have issues at some point or another. AKs, being a weapon designed to smile at fouling and ask for more, will simply run alot longer before needing a cleaning. The Piston setup will only help to delay the inevitable a while longer than a DI, which vomits into its own mouth <img src="/ubbthreads/images/graemlins/barf.gif" alt="" />.

If you have your heart set on a piston, and have the coin to throw at it, the ACR is more akin to an AK than an AR. I have found that I can see daylight between the bolt carrier and the upper reciever wall through the magwell. Or a good quality AK in 5.56 might serve your purposes well too. My Arsenal SLR-106F is a tremendously accurate rifle, since it's so well built. It was cheaper even than a base AR to start with as well.