A heavy slug load in a light (like 7.5 pound) shotgun will produce recoil more akin to a .470 Nitro Express than a .300 Magnum! However, since the .300's MV is a lot higher, as noted by Ridgerunner, the recoil of the rifle will be a lot sharper! (It hits you in a shorter time period.) For example, when I first started shooting a .30/'06, lo these many years ago, I preferred to shoot 180 grain loads as opposed to 150's, although I knew in my mind the 150 had less total free recoil than the 180. But it was applied in much a shorter length of time, hence was less pleasant to shoot!!
Keep yore powder dry!!
"Bitte, trinks du das Wasser nicht. Dahin haben die Kuhen gesheissen."