Reducing powder charge will naturally reduce velocity I sometimes make mouse fart loads ( 12gr 700x and 1 oz shot)for kids to use. They require some fiddling as the powder column is shorter and affects the overall length and have found them easiest to cut the petals off and roll crimp.
These are shot loads by the way and havnt been chronied but they have piss all recoil and sound like a champagne cork. As the weight of shot is the same as your slugs and the bearing area on the bore is approximately the same results should be similar.
It actually takes very little force to push a wad down the bore the choke being the only real constriction, I know this from experience but don't ask me how.
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