... how bout a converter that used .24 shot and a primer, then you could cast.
Problem with a heavier projectile is you need some poop to get it moving. The large pistol primers the convert-a-pell use don't even get a .177 pellet going that fast. I never put one across the chrono, but I'd estimate they're doing less than 500 fps.
Going to a 209 primer would help, but the barrel sleeve is held in place by the friction of O-rings. Even with the standard kit in .177, the barrel moves out a little after every shot and has to be pushed back into place.
I could modify my shell adapters to have the pellet seat against the primer (as designed, the pellet sits at the front end). This would give it a supported run up to the barrel at the cylinder gap and accuracy/velocity would be better. Problem with that is the increased pressure might blow the pellet skirt out, and any leading inside the adapter would be hard to remove.
Those problems might be fixed by using a ball instead of a pellet, but then you've gotta think about the ball not seating securely and rolling away from the primer.
In other words, I don't think the thing is worth messing with...
I have what I believe to be a better alternative I designed years back but never made - A .25 cal rifled airgun barrel blank could easily be machined into shell adapters similar to the ones provided with the convert-a-pell kit. Hornady #3 buck shot is .250" +/- .002" diameter, very spherical, and engraves perfectly when pushed into a 25 cal airgun barrel. With a 209 primer behind it you would have something fairly accurate and powerful at close range without need of a separate barrel sleeve. The ball would stay put because it's stuffed tight into the rifling upon loading from the breech end. The bore in the shell adapters would be straight through with no flash hole; a 209 primer is .250" diameter. That would make for easy cleaning.
Another 'survival' advantage to the above would be the ability to add a pinch of whatever powder you happened to have handy. The basic design could be adapted to just about any cartrige profile over 35 cal for any handgun, rifle or shotgun.