Remington used to sell some .22's, not sure what all models, with a special bore for the shotshells. They called it a Routledge(?sp) bore. As I recall it had about 6" forward of the chamber at regular size & then was enlarged to about 5/16" (3.125) dia for the rest of the length with some amount of choke at the muzzle. It was supposed to shoot the shot loads real good (I never owned or shot one). This should not be too hard for anyone with a good lathe to do, assuming they know how to use it.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra