I have a 12.5” midlength upper that has been a booger to hot rod up.  I could go mil spec-ish easy, but I have been trying adjustable gas, Low mass BCG, SCS etc for 3 gun shooting….all while dual purpose both suppressed and not with a YHM ULT.  The thing has been picky and runs super dirty.  I now debate starting over as a piston upper.  12.5” mid is a very short dwell time combination which is why its so picky.  I just saw a 14.7” rifle gassed piston upper running a few weekends ago so apparently pistons don’t need much dwell.  After all, the expansion chamber is right at the gas port in the barrel.
 
The only two Piston systems I know of are Superlative Arms and Adams Arms.  I want a lightweight Bolt Carrier and see that at Adams.  I also want gas adjustment and to put the thing under a skinny keymod rail if possible.  It would also be nice if the gas had multiple fine increments for tuning.

Superlative arms looks like it would fit under many rails and could go through the nut to many different handguard nuts as long as I drilled out the correct hole in the nut.  I do not know now their adjustment works.  It does appear to have many increments though.  It has no low mass bolt carrier option, but I have grinders, sanders, and dremels for that.

Adams arm railed system is on sale for $200 right now, but it only has 100%, 50%, and off.  Can I add detents for different gas settings?  Can I sand it down in shape to a low profile like the XLP so it will fit under forends?  Again, I would have to grind and dremel the bolt carrier.  Question, would the forward bleeding gas be hitting my suppressor or mount (12.5" middy has the gas block just behind the muzzle)

Adams Arms XLP... looks a little bigger than the Superlative arms, but should fit under many rails.  100%, 75%, 50%, 25% and off gas positions.  Can I make detents between those?  Already has a low mass bolt carrier option.  very expensive.  never in stock.

what else is out there. What rails work with each?  I don't mind grinding a little clearance in rails.

I remember reading and seeing pictures of a lightweight carrier in development from Superlative Arms. But that was like a year ago and I haven't heard anything since. I hoping they come out before I get my tax stamp back for my SBR.

I have two Adams Arms piston kits. First was one of their early models, 223 16" carbine gas, has functioned 100% from day one. The second is their XLP, 223 mid length. It is teamed with their upper receiver and 16" Gov't conture Voodoo. The XLP had a hiccup, last Thursday and the warranty replacement arrived today. Accuracy with the Voodoo barrel is excellent.

I was told they are releasing another version of their XLP around August.  I will not hesitate purchasing a third

My AA upper has run flawlessly for the ~2500 rounds I've put through it.

The Miculek muzzle brake compliments it nicely

The xlp does give you more fine tuning of gas settings and should fit under most ff rails. I think they used to have a list on their site of known rails that fit over their system.

https://imgur.com/gallery/yHTUr

I did a DIY conversion to the Adams Arms with an XLP gas block. Took a little bit to tune the gas and break it in, but after a hundred or so rounds the recoil is negligible, the action is flawless, and there is always a a neat little cluster of brass at the 4. Absolutely will do this to every AR I buy from here on.

I'm at about 1500 rounds and it is the smoothest shooting gun I've ever owned. I won't ever go back to DI.