If you are determined to get a piston upper, I would say just get one that is factory built. Some of the drop in piston kits for pistol length really have jamming issues, as they don't have a spring in the linkage to slow the cycling action a bit. That is the problem with AR pistols in general, DI or piston. The shorter the barrel, the more stoppages you generally have. The entire cycling action (to include the gas system and buffer) was designed from day 1 to work with a 20 inch barrel. That gives the gases time and room to dissipate the pressure and heat. I could go on, but I have strayed too much as it is. Just my .02. There are actually some AR pistols that work quite well because everything is matched
"Democracy is two wolves and one sheep voting on what to have for dinner.
Liberty is the well-armed lamb contesting the vote." - Benjamin Franklin