I actually made a few a long time ago,
Why ? as a garden gun adapter to kill pests like the plague of Rats that was attacking Me at the time.
I made it on a lark, but oddly enough, I was shocked to see how accurate it was, I could kill Rats out to 20 - 30 feet.
Your best bet is a 3 inch chamber shotgun, and use a scrap bit of .38 special Revolver Barrel from a gun shop, I turned the barrel down on a lathe a bit, fit it to a sleeve, press fit, to 12 gauge diameter, then I used a chamber reamer to make a chamber and throat.
One added benefit was it was how quiet it was in operation.
The sound in the backyard was more like dropping a heavy phone book, not like a gunshot at all.