To the best of my knowledge no one has ever made a mould for "rifled" slugs. Many years ago Lyman made a die that would "rifle" one of their standard foster type slugs. It was discontinued because it was of no value - slugs shot about the same with or without the "rifling" on them.
The rifling on factory loaded slugs is primarily a safety feature, it allows them to be bore filling yet be swaged down if shot thru a choke. Rifling on a conventional slug will not impart any meaningful spin as there is not enough of surface to catch air and make the thing spin.
There are several options for made-at-home slugs. First is the round ball, sometimes called a "pumpkin ball". Lyman makes several sizes. The next is the Foster slug, similar to the common store bought slug. These are hollow base and rely in the "rock in a sock" effect for accuracy. Lyman makes moulds for these in 12 & 20 gauge. Lee makes a version that they call a "drive key" slug, it is similar to the foster except that it has a bar of lead that bridges the hollow base, this bar is there to keep the wad from shoving into the hollow base. The last lug mould that I am aware of is the Lyman "sabot" slug, it is similar in concept to the foster in that it has a hollow base, however it is designed to be loaded into a standard one piece wad, and the slug is flat nosed and looks like a giant air rifle pellet.
I have never used the Lee slug so I will not comment on it, other than to say I really ought to order one sometime.
I was never terribly happy with the performance of the Lyman Foster type slug; they are generally undersized for most barrels. Plus they require fussy combinations of card wads or require other wads to be modified.
Round ball slugs are fun, easy to cast and easy to load and provide decent accuracy at close range.
The best home made slug is the Lyman Sabot slug, IMO. Accuracy in my experience ranges from good to excellent. Best results that I saw were from a friends 11-87 Remington with a rifled choke tube. Groups fired with this combo were so good they were almost hard to believe. If I were to quote group sizes people would think I was full of ####. The same load also shot decently from a plain barrelled 1100 Remington. A nice thing about the Lyman sabot slug is that they are easy to load, no special hard to find wads are required and your standard crimper works fine, no need for a roll crimper.
There are no doubt other custom or semi custom slug moulds out there (NEI makes or used to make one), but the above are common and may be readily purchased.
