#1 Seating primers flush is all that is required. I do not use Remington rifle primers but have not heard of them being too big before.
#2 SAAMI specs are maximum. Everything has a tolerance. Did you bur inside case mouth? If your cases are not long they don’t need trimmed. Don’t make it harder than it is.
#3 First of all I would not load ammo for anybody, till I was sure I knew everything and I will never know everything. None of us is perfect and if something went wrong I would not want someone else to pay the price.
What we know is that bullet/powder/primer combination is not good in your brother in law’s gun. Have we tried it in another gun? Assuming you started low on the powder charge, a little more pressure (powder) might just be the ticket. A slightly higher pressure load will expand the base of the bullet for a tighter fit in the bore. If it does not, then try a different bullet.
SAMMI specs are not wrong, bear in mind these specs are for manufactures and are for all types of bullets. .452 is recommended by most loaders for the best accuracy with minimum leading using cast bullets. .450 or .451 will work they just might increase barrel leading.
Here is my concluding question. Everyone stresses to check and recheck, measure and remeasure in order to avoid mistakes. How can this be done with such variation in factory equipment and factory specs? Am I missing something? Help please.
Don’t kill yourself over this. It appears your measurements are within tolerances.