.50's are pretty small. The lyman book gives loads for 20 bores, using shot cups with .54 calibre grooved bullets. Yes- the bullets inside the shot cut. I assume they are suggesting this useage in rifled barrels.
; The guys I knew who used 20 bore balls inside shot cups were shooting them from their muzzleloading shotguns as well as their choked duck guns. As well, some took to cloth patching a .680" ball for use in choked shotguns as well as thin cloth patched .715"RB's in cylinder bored guns - all loaded inside plastic shotshells. The ball whould pass the choke just fine and hold about 5" at 50 yards with just the bead for a sight.
: Measure your choke and measure your bore back from the choke. Too- you must be able to push or tap the ball with cup out the choke - it will/should be VERY tight, but you should be able to do it. That is how you get the right size for shooting balls in a choke. If it's too tight, it may ópen'the choke somewhat- same scenario as shooting steel shot in a gun not designed for it. With double guns, you could loosen the ribs as the muzzles try to expand.
: Most full chokes have 40 points of choke. ie: .040" of choke - .020" per side.
; So- in a 16 bore, IF the actual bore measured .662", you would have very close to a .622" bore with a full choke.
; Measure the choke and that is the size ball you CAN shoot if you want. Of course, you can shoot anything smaller as well- but - smaller means lighter and more critical of wads to take up the slack and keep the ball in the middle of the bore. If it bounces up the bore, side to side it wil be very innacurate, just as the round ball loads were in the early 'pumpkin ball' loads. They used .63" and .64" round balls in a 12 bore - these had .050" or more windage, almost 1/16" too small for a choked gun.
; I sugest you buy lead or steel wads and cut the gas checks off the bottom and use those as I've suggested before as they centre the ball in the bore and shoot with improved accuracy over loose balls.