Leaving the chamber in the barrel can do some interesting things. I build my young son a miniature trade gun twenty years ago from a .410 cartridge barrel. One fine day my friend Tall Pine, and I, decided to go out shooting trade guns. My son Ian ( then six years old) begged to go along, and shoot his trade gun. He had shot round balls out of it quite a bit, but never shot. I explained the principles of shooting clay targets while Tall pine ran the thrower. When the bird was released Ian followed the bird just as I instructed, covered the bird with the end of the barrel, and swung through it an fired. The only problem was by the time he had done all this, the bird was about 35 yards out there. At the crack of the little gun the bird ( that was already falling) vaporized. Tall Pine, and I, stood there with our mouths open, not really believing our eyes. Finally Tall Pine looked me in the eyes, and said "its a full choke". I said it couldn't be I cut over a foot off the barrel. He measured the muzzle and scratched his head for a while, then asked me if I cut the chamber completely off the barrel before breeching it. I said no I cut the threads off the old barrel and left the remainder of the chamber. He said it was working like a jug choke in the short barrel. I said it couldn't. He said load her up. I did, and handed it to him. Now you got to understand Tall Pine is about 6'4" tall with the longest arms I've seen. I couldn't help but laugh watching him shoot the little musket. I pulled the string, and the bird flew, and a moment later there was a bang and the bird blew up. He handed the gun to me and said "its a full choke". I then loaded the gun with the same charge but added three more cushion wad to space the shot column out past the chamber, and sun of a gun it shot like a cylinder bore. This tells me a jug choke can be quite a ways down bore and still work.
Hungry Horse