A friend approached me with an idea for a pellet gun of sorts. He had an old Crosman .177 cal BB gun barrel that was about .430" OD and 10" long. He was thinking about chambering it for some small caliber and driving pellets with a primer alone. I had an old .44 mag Contender barrel I've shot 20 rounds through about 10 years ago and it's sat since. Turns out that the nominal bore dimensions for a .44mag is .417". So I put the Crosman bbl in the lathe and turned the muzzle to .4165", the breech to .413" and ±.400" in between.
I made the breech .413" so I could counterbore a piece of .44 Magnum cartridge brass with a Z letter drill and glue the Crosman barrel into it. I also drilled/bored the primer pocket to .243" to hold flanged 209 shotgun primers and chamfered the primer pocket to clear the 209's inside radius between the flange and the primer body.
So it's not my prettiest work and this was just a proof of concept. Testing supports that the concept is valid except that standard thin/soft pellets will be stripped of their skirt and the head will be ejected. So the concept might be more appropriate for .22 caliber barrels or a more robust pellet can be cast or purchased. It is interesting to note that the .177 pellet head alone will penetrate 1/2" of pine 2x4 at 3' standoff; perfect for plugging popcans in the back yard.
Improvements are to thread the Crosman barrel breech end and machine a permanent steel flanged piece for headspacing the barrel and the 209 primer.
An extractor for the primer should be made since the dimensions of 209 primers-on-hand vary and might become an interference fit. A positive extractor would make 'reloading' much easier.
.22 and .177 Crosman barrels are available for $20 or less on fleabay and come in 7", 10" and 14"+ lengths and they have real rifling last I checked.
Anyway that is how I entertained myself for an a couple hours last night.