I have had .308 Carbines in barrel lengths from 16" - 20"... IMO, 18" is the best balance of compact handling versus feel and performance. 18" is short enough to sling over one shoulder and duck branches without catching the muzzle, you can swing in tight spaces without hanging up, but you also still get good balance and enough velocity to make longer shots if necessary... it makes the rifle more versatile... it probably also will maintain a little more value and appeal for the general market over a 16" barrelled version if you ever end up selling it. I have had 16" barrelled .308 rifles in the M77 MKII Compact and M77 MKII Frontier rifles, and while they are handy, they seemed unnecessarily short when put into use in the woods. A 20" carbine is an option that you probably find satisfactory for handling, with better performance for longer shots... but if you are busting a lot of thick brush where you do a lot of ducking, I would fall back to 18". What platform are you building your carbine on... or are you just cutting back a rifle? If you are bobbing, you are limited to 18" anyway.