OK, this something for a kraut, take Magtech 16 ga brass, aneal the case mouth( its very hard) and buy a RCBS 16 ga shotshell die set.
The RCBS die have a shell holder and a die body holding all parts:resizing die, roll crimp die and other parts you dont need here. If you need wads,I recommand cicle fly fiber wads. not 16 ga. for Magtech brass, cicle fly know this problem and will send you the rigth one. Start with a roundball or another slug you have.
Look at the length so you hold the slug in the case with the roll crimp, it wan't rattle.
You can buy 16 ga brennecke slug in krautland, ask Ballistik Products if they have them in the States. If you have problems that the wad is to small for the magtech brass case, also with brennecke slug base wads, patch with a piece of paper like a muzzle loader roundball. this helps!