I have used just about all brands of lowers, a LOT of them come from the same machine shop but with different brand stamps on them. They are all Mil-spec and you should not have any issues with any of them. In My experience the RRA and bushmasters fit tight on most all uppers, the DPMS and armalites have the most slop between the uppers and lowers. I have an M&P I am buliding now and it is about equal to the Bushmaster and RRA in fit and finish. I just 0rdered 3 more lowers for $85 each with a group of guys that placed a big order, but we have about a 4 month wait to get them.
"A free people ought to be armed" George Washington 1790
‘For those who would give up essential liberty to purchase a little temporary safety, deserve neither Liberty nor safety.’ —Ben Franklin.”