If it is a custom rifle then there are 2 other things he needs to find out.
1. Was it reamed with a tight neck? If it is a benchrest rebarrel then they generally do. The brass may have to be neck turned in order to chamber. Way to tell:
a. Measure the outside diameter of the neck on a loaded round
b. Measure the outside diameter of the neck on a fired unsized case
If the loaded round dimension is not .003" LARGER then neck turning is a consideration, if the loaded round dimension is less than the fired unsized case neck, then outside neck turning is mandatory
2. What is the barrel twist? If the previous owner wanted to shoot the heavier bullets then he would have put a faster twist on the barrel and that will greatly effect what bullet the rifle will shoot.
Lots of questions, need more info on the rifle, magazine length and throating.