One of the more interesting guns from the venerable firm of Crosman was their Trapmaster 1100 shotgun. Powered by a pair of 12 gram CO2 cartridges and firing a reloadable .38 caliber plastic shotshell filled with #8 shot, the Trapmaster wasn’t a huge commercial success, being made only from 1968 through 1971. It was sold not as a hunting gun, but as part of suburban trapshooting set, using a hand thrower and two-part, snap-together plastic “pigeons.”
Of course since it was made, airgunsmiths have found it to be an excellent platform for customization. 1100s have been converted to bulk fill and re-valved to increase power. Changing the valve spring or making a new, lighter, valve would increase opening time and the the volume of gas released. The guns can also, with no conversion, fire a .375″ lead ball over a couple of wads. One owner, writing in (I think) the now-defunct Airgun News, claimed 100 ft-lbs of muzzle energy for his conversion. A more realistic power level of 42 ft-lbs (465fps with an 80gr ball) is claimed at this web site. Whatever the capabilities, the 1000 was one of the best made, best looking guns Crosman ever produced, with a solid heft that put you in mind of a medium caliber firearm.
I owned two at various times, back when you could buy one for $70-80. These days a good example can command as much as $300, and a complete set with trap, shells, and reloading kit can go as high as $500.
David Bartle very helpfully added these shell dimensions in the comments. I thought enough people would find them useful that I’ve copied them here.
Crossman Trapmaster 1100 shell dimensions:
Small outside diameter: 0.435 at top
Small outside diameter: 0.440 at bottom
Large outside diameter: 0.622 (flange)
Small diameter length: 0.682
Large diameter length: 0.930 (flange)
Inside shell diameter small end: 0.368
Inside of rear wad inserted in diameter: 0.354
Inside depth of wad from flange: 0.254
outside depth of wad from flange: 0.142
Length overall: 0.780
Measured from unused shell.