Bonehill, I had a shotgun with no choke in the right barrel. Mike Orlen jug choked the gun for me.I didn't spend alot of time recording pattern tests but it did shoot tighter with the jug choke. My understanding is that the length of the jug only has to be as long as the shot column in the barrel to have the same effect as a back bored barrel.The barrel wall has be thick enough in front of the choke to allow some metal to be removed.
Good luck, Dave