I have never fired a .300 ultra but have fired the .300 win mags. My Finnlite in 7mm rem mag is a very comfortable rifle to shoot. I think the .300 WSM would be fabulous in the Finnlite. You realise that they now have an action designed specifically for the short mags. The Finnlite will weigh about 7 1/4 lbs naked so it still has some weight. Go for it, it will become your goto rifle for everything.