This one actually exists, you can buy it today and it's not real expensive, plus you don't have to buy anything you already have. It's not a NFA weapon unless you want to cut down the barrel. Say if you have three 12 gauge pump guns you can buy a 20, and then get 16ga, 28ga and 410 barrels. At that point you will have everything worth having in a smooth bore. It is the single shot NEF shot gun. I think they are about $100 for a complete gun in one gauge and additional barrels were $36 each back in the day, I'm sure the price has gone up some over the years. Only inconvenience is you have to have the barrel fitted to the gun at the factory. It's the cheapest way to get into a new shot gun gauge with a gun that is 100% reliable. They are single action only, auto eject the shell when opened, have no safety and are pretty light weight, you may not want to get one in 12 and/or 10 gauge. The 12 gauge only weighs 4.5lb and it will pound you when you fire mag shells.