Newbie here, don't flame please.  But as the title shows, is there any problem with mating an RRA varmint upper with a BM lower receiver??  The reason i ask is because i have a bushy lower right now.  I'm looking to build myself a little 16" all-purpose coyote weapon.  This includes engaging targets up to 350 yards, and also somewhat closer engagement in the thick stuff.  Anyway to get this upper in a fluted barrel??  I don't play that "you gotta get yourself a 24" yadda yadda yadda.  Forget that.  Only Tom Beringer needs a damn 24" heavy pile like that.  I'm looking for a lighter do-it-all weapon that will  shoot MOA and thats what the 16" will give me.  Any other suggestions are welcomed.  Thanks

Is there any problem with mating an RRA varmint upper with a BM lower receiver?

No problem whatsover.