I have absolutely no experience with a piston setup on any AR platform rifle, but 4227 is all I've used in my gas 300 BLK. Can't find anything else in my area to try right now. I haven't loaded any 125 grn. bullets either, but I've loaded about 200 110 grn. bullets with 19 grns. of 4227 and they cycle fine in my 10.3" bbl. with the pistol gas system and my son's 10.5" bbl with pistol gas length. The 19 grn. load is a compressed load and the Sierra load table shows a max of 20.9 grns. for 4227 but it's beyond me how you could even get that much powder in a 300 BLK case! Sierra shows a minimum of 16.2 grns and max of 18.7 grns. of 4227 for the 125 grn. bullets. Don't know if this helps, but it's all I have.