I make quite a few slug molds, some up to 1600 grains, but only for rifled barrels.
I suggest you check out the above recommendation about Ballistac Products. They are unknown to me, but I don't make hollow base slug molds, and for smoothbore, I'm convinced they are mandatory for accuracy. Choke has made no difference my slug use through the years, so far as danger, but it seems that modified or cylinder tubes have shot most accurately.