6.5Grendel

good evening ,is modifying live/loaded factory ammunition lawful

I don't see why not , people have been pulling the bullets off military surplus and using decent bullets instead for years.

I guess there could be an issue if you change a target round into an expanding one without the condition on your FAC . Rimfires don't have to have expanding on ticket as far as I am aware although I have never had just rimfire on ticket so could be wrong. I have always assumed that rimfire ammo , particularly .22 lead ammo , will expand.