Because the opportunities to swing a gun very far are significantly constrained when hunting grouse and woodcock (unless you walk trails all the time), how well a gun swings for you is not all that important. I agree with the above post that barrel length, in and of itself, is not the real issue; also agree that it will very seldom be the last couple inches of the barrel that will contact a tree as you mount and attempt to engage the target in question. But if you're carrying one-handed by the grip, the longer the barrels, the more weight there will be in front of your hand vs behind it. That may make a difference when it comes to one-handed carrying. Of course one solution, which many grouse and woodcock hunters follow, is to go with a gun that's both short and light.
Had something interesting happen the other day, when I was responding to a flushing woodcock. When I moved my left hand to grasp the barrels, I also grabbed a small branch from an adjacent tree. Didn't have to make much of a swing, but I'm blaming the miss on trying to swing the tree in question in addition to the gun.
