I load the Nosler 95 gr. Partition bullet for a friends wife to shoot in her Win. 243.
A 95 gr bullet in a .243 is a "normal" weight. A 100 gr bullet in a .260 is undersized (i.e. shorter than normal). That is going to have an adverse effect on both the BC and sectional density. The lightest factory loads I've seen in .260 are 120 grn.