I'm a big muzzleloader and I cast a .600 round ball for use in my .62 early Virginia rifle. I would like to use this ball to load up some slug loadings for my 20 gauges. The ball weighs 325gr or 3/4oz. Does anybody have a starting point for such a loading? I have plenty of filler wads to take up space so it doesn't matter if it's a 2 3/4" or 3" loading, I can do both.
I'd first advise you try those balls in the muzzle of your gun, if it has a choke of modified or full they will not fit and should not be fired. I've done some shooting with roundballs of .562" loaded inside a plastic shotcup and have gotten fist size groups at fifty yards and could keep them on a paper plate at 75 yards. A .562" lead balls weighs a bit less than 5/8 ounce and there is no data published for such a light load in 20 gauge. I gradually worked the charge up to 15 grain of Red Dot and pressure still seemed very mild. I reloaded one hull over and over until I got tired of it. I never chronographed them but accuracy was good and they seemed to have power enough for short range hunting. You can't use a shotcup which has any ribs inside, they won't allow the ball to seat down to the bottom of the cup.
Your .600" ball will weigh just shy of 3/4 ounce and there is data for 3/4 ounce shot loads. If they pass freely through the choke of your gun you could load them with card and fiber wads. Why not try them with about 3 drams of blackpowder? That would be much safer than experimenting off in the blue with smokeless powder which often does unexpected things.
I don't know your reason for wanting to do this. My reason was to find a deer load with not such brutal recoil as the factory slug loads but with equal or better accuracy and for me, the roundball in a shotcup does that. Recoil is quite pleasant even from a five pound singleshot.
