The Vang Comp system is closer to (but not) a very lengthened forcing cone as opposed to a choke. The forcing cone on some factory short barrels is tiny, which causes more disturbance to the shot as it travels down the barrel, when the shot becomes damaged the group size grows. That's why you see plated shot and special wad designs, it stops the shot from getting bruised. Vang Comp tightens the pattern without constricting the barrel. It's more effective with regular shot than it is with plated shot for that reason.