>>64103457It matters a lot, especially with 5.56 which is designed around a 20". More complete powder burn, less muzzle flash, higher velocity, more range at which the projectiles fragment reliably, longer accurate range overall.
You can get some acceptable performance out of an 11.5" with boutique ammunition, but you are going to be paying a fortune for it. Running that with standard M855 or M193 is going to result in a lot of random ice picking beyond about 100yd. Of course that limited range can be worth it for for the shorter/lighter rifle as long as you are only going to be fighting within short range anyway, but that does gimp you a bit if you need to reach a little further.