I have a "Rhino" short stroke gas piston on mine right now, and I LOVE IT.
You can fire a 90 round drum and take the bolt out and lay it against your arm!
The other AR-15 is an HK top end with their new short stroke gas piston kit on it, and it's just AWESOME, but it doesn't have NEARLY the adjustability the Rhino kit has.
It has a piston you can flip over for different types of ammo, but no control valve so you can tailor the gas charge to the piston...
No bolt/bolt carrier erosion, (a REAL problem for those of us that fire a lot of rounds)
No gas tube erosion or plugging,
The bolt/bolt carrier doesn't heat fatigue,
And since you can dial the amount of gas that gets to this one, you can fire MUCH 'Hotter' rounds without bolt bounce or short stroking,
Or the most dreaded cycle failing, extractor ripping the lip off the brass!
Opening the gas valve all the way up will get your rifle to cycle even with subsonic ammo in it, so I'm VERY happy!
Cleaning take 5 to 10 minutes now no matter how many rounds you fire!
No blow back in the receiver (unless you are using a suppressor and full power rounds) so cleaning is a snap and no waiting an hour for things to cool down enough you can work on them!
Last KY machine gun shoot, after about 2,500 rounds as fast as guys could load the thing, and I was clean and cased and holding a beer in 15 minutes!
Just drove the guys I was with crazy!
Their rifles hadn't even cooled down enough to handle the bolts yet!
I have two AR-15's and one AR-10 with gas pistons, and I'm looking for another AR-10.
Seems all of Rhino's production is going over seas to our special forces right now on the M-110A sniper rifles, so the Special forces guys think they are the cat's ass too!