I'm gonna throw my thoughts on Pistons and the AR platform in general in...
In essence, my feelings are this:
On the night that E. Stoner sat in a corner of some bar nursing a beer and doodling on the back of a bar napkin, rifles with pistons were nothing new. The G3 existed, as well as the FAL among others. Now we all widely acknowledge, E. Stoner as a genius. Thus, it's not for me to second guess the man. If he chose to go with a direct impingement system, he probably had a reason far beyond my feeble mind for choosing it. Thus, if I want a piston gun, I'll buy one in which the weapon system was originally designed with a piston driven action.
I've handled LWRC rifles and they seem nice. I've read good and bad but, as you may have guessed, I've never bought one. :wink:
I think Mr. Stoner designed a rifle system, in that the bullet, powder, gas port size, gas port location, rifling twist, carrier mass, buffer mass, spring rates, barrel length, etc, all worked harmoniously. Ordnance messed up the powder part of the equation, as is well known, and early rifles issued with minimal instruction were not cleaned correctly.
Then, Colt chopped up the design to create shorter versions of the original M16, leading to all sorts of design modifications to deal with modified pressure curves and other changes that initially "broke" the balanced Stoner DI design.
Piston ARs are a direct response to the shortcomings of the M4 and other short barreled M16 variants. IMHO if Eugene Stoner had known the M16 was going to be converted into a bazillion different SBR configurations, many of which are shot with suppressors mounted, he might not have chosen DI for the operating system. This is why the emerging consensus is that pistons are really only needed if you shoot very short barreled AR configurations or shoot suppressed. A plain-jane 20" A4 does not need a piston.