What twist rate is the barrel on his rifle? If it shoots good groups with 162s its likely something fairly quick like 1 in 9.5 or 1 in 10. I shoot 168's and 180's out of my 1 in 9, but have loaded some varmint bullets using hornady 100 grains. Using a light bullet that is overstablized in that fast of a twist is not ideal. Dont run them up to hot or accuracy will do down even further. You need a 1 in 14 or so if you want a real varmit gun in a 7mm mag. For me my usuall 3/4 moa three shot groups with 168s went to about 1.25 with the 100s. Still more than accurate enough for hunting. Those weatherby rifles tend to have long throats so you might really be in bad shape with 100grain bullets in terms of not being able to get them seated out close enough to the rifling. Having a large jump will hurt your accuracy as well. Load up a box and see what they do. THe hornady 100s are dirt cheap at about around 20 dollars for a 100 bullets.
