My M1S 16" CAR upper ( the heavier barrel) is run on a DPMS based Clark Gator lower.
Other than a certain finickiness with regard to ammo choice, It runs fine. For high volume fire, I'm using Wally Fed Value Pack 550 in BDM full body or skinny mags.
Certain ammo feeds mainly from the skinnies.
JAC steel mags feed anything.
My best accuracy was about 3/4" @50yd, but I don't remember what ammo.
My regret was that when I picked up the upper at the gun show ( custom assembled per phone and fax request-flat top 9mm receiver, 16" CAR, CAR handguard, regular front sight) I got a BDM skinny included (along with the modified chamber adapter piece- since I already had the JAC conversion) instead of the 10 round Ciener, which would have been the same price. Since the JAC 10s are $45 and BDMs are $30-35, I screwed up.
For any kind of bench work, the JAC 10 rounders are great, since they only stick out about 2" below the mag well. High capacity mags and benches don't mix happily.
I am very happy with my M1S. I understand Spike's is excellent also. I have some of his " .22LR" marked lowers ( not yet built up) which seem to be of excellent quality.
I also hope to build a featherweight more accurate upper. I am reflecting on whether to send a Lothar Walther barrel to be turned to light CAR contour, versus a Tac Sol Ciener compatible stainless lined aluminum, vs a double throwdown super duper aluminum sleeved steel barrel I hear Walther offers. I plan to use a Clark graphite handguard ( light, stylish), and maybe a carbon flat top upper, and mount the whole thing on a Cav Arms lightweight lower, saving more weight by leaving out the spring and buffer.
I would be interested to know the weight of your upper.