Originally Posted by KRP
I would determine the cause of the excessive headspace first.
Except for swapping out the original barrel, bolt or firing several extreme high pressure loads that set the locking lugs forward, there's only one reason for excessive headspace in a rifle; too much space between the bolt face and the chamber's headspace reference point. That's caused by reaming the chamber too deep in the barrel's breech end.
If ammo that's ready to fire has too little case headspace and having excessive headspace when chambered, that's caused by the case having too little space between the case head and the case headspace reference point. Excessive full length sizing that sets the fired case shoulder back too far will cause this with rimless bottleneck cases. For all new cases, a bad lot of brass will cause this.