I'd like to buy an AR. No real reason except "just in case", I guess. How come virtually all of the AR 15 makers only make 16" barrels ? How come nearly all of the AR 10 makers think 18" barrels is the right length? I think I saw a 20" barrel on a $3000 AR 10 unit. I am sensitive to muzzle blast and think 20", maybe 22" is about right for the AR 15 and NO LESS than 22" for an AR 10. I've been watching for one of these apparently rare critters but anything over 16" (5.56), and 18" (7.62), seem to be in the $2000+ range. How come? It can't be all that more difficult (bit of sarcasm here) to fit and chamber a longer barrel in these rifles. I'm both disappointed and depressed I seem unable to find one if these critters!!