Since you can't have a rifle barrel installed, just cut down the shotgun barrel. I cut down a 20 ga for my son, and it is one of the easiest guns to carry around. I lost the choke by cutting it down, but it increases the odds of my 7 year old hittng water bottles with it. I've got a cut down 45-70 that I keep in the 4wd, but a 12 ga would serve the same purpose.
Jon