What stock are you running?
If full length then try a CAR spring first & advise of results.
If running a CAR stock then M1S has a 300-221 package w/a lightened spring & buffer that works w/subs ONLY.
HOWEVER, DO NOT RUN MANY SUPERS THRU W/THIS COMBO AS IT WILL BEAT YOUR GUN!
I tried some #9 with 240gr SMK's with no luck. I tried 1680 next which functions and is SS out of a 16" bbl with a rifle buffer/spring/tube. I have not got to shoot for accuracy yet due to the fine weather we are having.