I have a buddy with a 300 WSM. That gun loves the 165 gr. fusion ammo. Under an inch at 100 with no real rest. He shot a 370" bull elk last year with that gun and it was DRT. Bullet performed great! Shot was a tad over 350 yds.
Based on that and other friends killing muleys with the same brand, I would not hesitate to use fusion bullets if I shot factory ammo. I also really like the nosler silver and balistic tips especially the 180 gr. versions. They shoot and perform really well.
Back before very many barrel makers knew how to cut a stainless barrel hart made most of the good ones.
When Remington came out with the 7mm rem mag they had trouble with the barrel throats eroding and found that if they used stainless barrel life was improved. The problem was they couldn't make a quality stainless barrel so the commissioned Hart to make the stainless barrels for them.
If you look on the barrel there will be a Hart shaped proof mark in with all of the other proof marks. Remington said this mark was just a proof mark (What ever) Hart did this just to protect themselves.
The custom shop used a lot of the hart barrels until Remington finally learned how to make stainless barrels. At one time all 40x rifles were made in the custom shop.