Well...You can bore out the ports to launch them as far as weight goes, but are limited to .1875" for the stock valve due to being too thin walled beyond that for safety. So, an aftermarket valve like the Cothran powerhouse valve would be an option, but it is a "dump valve" so forget any kind of bell curve unless regulated, which would be pointless over just a few shots at a reduced power level. Slugs typically need more speed to stabilize them. Then there's the choke to deal with, though after market barrels could be had, or you can cut the choke off and add in more baffles to compensate for the shorter length. You could tilt the slugs in if slightly to long, up to a limit. You didn't give the length to help us out there. There's one member that is slinging .357 around on the forums currently. Maybe look up what he is doing. I have no interest in that so have no clue what he is doing there. I have never heard anyone put in a good word on the Seneca's'. Plenty of negatives, IIRC.