I have run into that quite often even when rebarreling a 700 with a standard barrel and the factory lug. If you make the barrel fit the action threads, the lug will not fit. The proper way to fix it is to chuck the lug in a 4 jaw and bore it but a "field expedient" way is to open the lug hole with an emery cloth flap wheel or emery cloth wrapped dowel. When it fits, clean both sides of the lug with a stone or fine emery cloth on a flat surface.