For the shot gun barrel you might grt a small cycender hone and hone it out a bit. mike it before cutting and hone it to where it should be. when you cut the barrel it will shoot very "loose" because the barrel is tighter. ie, a looser barrel has a tighter pattern. It has to do with gas leaking around the shot and pushing it together tighter and surrounding the shot as it leaves the barrel. honing the barrel is called swaging it.
good luck , should be pretty cool little gun when your finnished.
walt