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I have done three: an Adams Arms, a CMMG, and a Osprey Defense. If you want the most versatile, use the Adams Arms since it can be turned off or turned down for reduced gas. It is also very easy to clean since the psiton and rod come out the front and then get cleaned and reinserted.

The most trouble free is the Osprey Defense, with a piston cup and rod which are darn near self cleaning with modern ammo. I'm into the second thousand rounds with the Osprey and yet to clean it or have any glitches.

The CMMG is a good system, allows easy conversion to .22lr ammo, and cleans up nice. But of the three I would go with the Adams Arms for ease of use, or the Osprey Defense for easiest conversion. I have the CMMG on an AR pistol with eleven inch barrel. It took a tiny bit of tweaking of the recoil spring to get it to run flawlessly.

All three of my conversions were on carbine length gas systems. I would recommend the Adams Arms for sure if you have a mid-length gas system ... most trouble free recoil action.