just picked up a 338 lapua 20in barrel in a light hunting stock, id guess about 8lbs. i was kinda worried about how hard it would kick being so light, but with the badger fte on it kicks less than my 12ga. with high brass shells. doesnt seem to change accuracy at all, and its built like a tank.
I have ran and still run the FTE's. Ran one on a tuned 700P in .308, recoil was non-existent and on par with a .223-.22/250.
Currently run one on a 700MLR .338 LM, recoil with the FTE is roughly the same as a heavy barrel .308 without a brake. I can sit for extended range sessions with the .338 LM and recoil is not a problem with the FTE.
Should work great on your .300WM and take it way down in recoil.
In my experience the FTE works great on magnum rifles,and works okay on 308rifles. 338 lapuas really benefit from an FTE and I think the 300 win mag would as well. My 338 experience is similar to da law dogs.