The heart of an AR-10 is the bolt carrier group, it's heavy. Next is the appropriate buffer, it's heavy too. Trying to balance it out with a light barrel won't work.
So go 20" and make it heavy enough to settle down good, it'll help negate zero shift when hanging a 1 lb. can off he end of it.
A handy dandy AR-10 carbean just doesn't work out well.
