Cadet;
My experience with the older type wads, though primarily with 12 & 20 ga's, was that the normal wads measured about 1 ga sze larger for 12ga & more for the 20ga, in ga size, but slightly less by measurement. Thus a 12ga wad would actually measure about an 11ga (.751"). On this basis a 10ga wad would go 9ga size (.803").
Thw wad of course wants to be a good tght seal in the bore, but must load easily in the shell without swelling it enough to cause hard chambering.
Without having actually tried these sizes I would "Speculate" the .812" (+.037") is about max for the 10ga with paper/plastic hulls & the 15mm (+.0405 @.5905") is most likely over the limit on the 28ga. Something on the order of 14˝mm (.570" @ +.020")) would seem more reasonable here to me.
