Hi.
Well, you will find that its not so much the length, as what you do with it....in a tight spot you will find the shorter barrel is a god send!!!
As long as your paint fits the barrel ok, then as short as you can go really. However, a longer barrel can be used to push into the bunker, so you can shoot easily but the rest of the marker/hopper and you are hidden behind it (on inflateable's)
You also get better gas consumption with a shorter barrel and not too many ports.
Being at the front mean being as tight a poss, but still remaining a force to be rekon'd with, its a balance.
Hope this helps.
Cheers
Gaz