I think the grooves were put there to "Insinuate" they imparted spin. Lyman at one point some years back sold a Foster type mold & a separate Die for putting in the grooves. I recall reading reading a test report by the NRA staff probably some 40 years ago in which they fired some of the slugs cast in the Lyman molds both un-rifled & rifled. Accuracy was virtually identical either way. This was still in the days when Card & Fillers wads were King & it was noted in handloading these slugs it was essential that either a .200" Card or two of the normal .135" cards be placed atop the filler wad next to the slug to prevent wadding being pushed into the hollow base & destroying accuracy.
Th e solid nose portion of these slugs were made small enough to go through any choke normally encountered with only the skirt expanding to fit the bore. This skirt would squeeze back down when going through the choke. The old Ithaca "Deerslayer" guns had a cylinder bored bbl having about the same diameter of a normal full choke for its entire length. These barrels were noted to have some of the best possible accuracy of any un-rifled barrel with the Foster slugs as they did not expand the skirt so much.
