Most of the domestic piston conversions I've seen for the AR15/M16 have utilized the FAL type system with many actually using surplus FAL parts as they were available and dirt cheap a few years ago. Basically just a new or modified front sight/gas cylinder is installed and the carrier key is replaced with a solid one for the gas piston to push against. IIRC, the "Rhino" system was probably the first, first saw it back in the 80's I think. It used their own newly made parts but in overall design it was very similar to the FAL without a gas tube.
Moving the gas system forward and enlarging the parts keeps fouling out of the bolt and carrier, reduces upper receiver temps during rapid fire, and allows more dirt to accumulate before stoppages start happening. A valid consideration for a grunt but less so for the civilian shooter. Very few civillians drag their AR through the mud and sand, do amphibious landings, or go days without cleaning while shooting offf hundreds of rounds.
I've shot a Rhino back in the day, can't say I noticed any difference between it and an unmodified rifle. I have no doubt that it would have continued functioning long after an original gas system would have been fouled to the point of malfunctioning. Since I clean my AR on a regular basis I find justifying the added $600-$1000 cost of the gas piston conversions pretty hard to do.
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