Not familiar with paintball myself, but I'd guess that continuing Eddy's point, a firearm has an excess amount of propellant thrust from the burn and is still accelerating the slug for the full length of the barrel, which then makes the extra length useful. In contrast, a paintball gun is working at a much lower pressure level and likely stops accelerating the ball before it leaves the barrel, even in the shorter length, so you're not gaining from it nearly the same result. Just mentalizing here.