That might solve the problem. I didn't know that was an option.
The difference might not be much, but my understanding is that with a longer barrel, the velocity is greater, the blast and recoil is less, the sight radius is longer for back up flip up sights, and bayonets work better on a longer stick. Short barrels might be better for carrying, and be less cumbersome in the brush, but that won't be an issue on the range, or anywhere someone wants to take a longer shot. If you want to clear a room, get an M4.