Re: 300 wsm issue
1. Custom or factory chamber?
2. Early reamers were often tighter than dies which has led to sizing issues with early reamers. So are even are using small base size dies on the early chambers. One of mine is like that, as it will not work with standard FL dies, but perfect with the small base die.
3. At 65,000 PSI in the WMS, FL sizing is a must. If the bolt is closing stiff, you are still probably a tad long on the shoulder. It should not be that tight. try another 1/16 inch movement around the die ring (not moving the down down but a couple .001)
4. IMO standard primers work better with the WSM.
5. H 4350 is a "go to" powder, but not normally the lighter bullets. RL17 seems much better with 168s
6. You need to measure the headspace and shoulder bump. Buy the hornady guage to do that or get a gunsmith to make a bump guage. I have the smith make a bump guage every time I get a gun chambered, as they use the chamber reamer to do it.
Nothing wrong with the group or ES, especially with magnum primers.