21" FTW.
Forget what the "compromisers" say
To compare this question to real, actual use. Consider recent events when our soldiers use MBR 7.62 rifle type autoloaders. This reference is to the M-14 type rifles you see in pictures in Iraq and Afghanistan. Our men are usually static, foot patrol/kicking in doors or mobile by truck.
How long are those .30 caliber barrels that were using in those pics?
Goes back to the same point I always make about FALs,AR 10s,M-14s. If you're gonna shoot .30 cal rifle rounds, most of the time doing so out of a Rifle is gonna pan out better than a Carbine.
Plus. If you get into some end of the world zombie attack. You'd most likely by trying to put as much distance between you and danger. So long shots may very well end up being the rule and not the exception.
Just like some of you may think clearing rooms with a 21" looks silly, I think you'd look like a damn retard not being able to hit shit at 600yds with your 7.62 sbr.