Depends on how much different they shoot. If they shot pretty close windage wise I would just sight in with the lighter bullets then see how it shoots with the 100 grn bullets. Chances are out to 200 yards there will not be much difference, at least not enough that will matter on a target the size of deers vitals.

I have been told though that most .243's do not like the 55 grn bullets, just to light and fast for the twist rate I guess. So you may have to go with something heavier anyway just to get it to shoot well.

I'm thinking of doing the same thing, but I am just going to split the difference and use a 80 grn or so bullet.

The trajectory for a 80 grn bullet out of a .243 is pretty similar to a 55 grn bullet out of a 22-250 out to 300 yards. Plus it would carry more energy do to the heavier bullet.

Paul