I have used a 12" disc sander for 45 years in the trade. with a pad jig.
)the 12" allows you to sand the entire butt area across a constant traveling surface. with smaller disc, your stock is half on each side (opposing travel and can make for some interesting movement .
having an almost 6" surface all moving in same direction gives much more uniform and flatter surface. I also use a B-Square recoil pad jig (still available on Ebay) JM.02

Brian
LTC, USA Ret.
NRA Patron Member
AHFGCA Life Member
USPSA Life Member