I've always been a big fan of 18" barrels on fighting rifles, both for 7.62x51mm and 5.56x45mm. With 7.62x51mm the ballistics performance isn't affected nearly as bad as 5.56mm when you go with a shorter barrel, but in both cases I feel 18" offers the best overall balance of compactness, accuracy, and ballistic performance. That, and it makes for a very attractive rifle profile, not that looks really matter on a fighting weapon. I've got an 18.5" barrel on my LRB Mk14 MOD 0 build and I'll be mounting an 18" 5R stainless barrel on my LMT MWS308 later this year. The same thing goes for FALs. The 18" "Congo Style" FAL with the side folding stock will always be my favorite FAL, and I think one of the sexist fighting rifles in history.