Before I had a proper framed pack I practiced sitting and using the sling as per Jack O'Connors directions in his books. Spent a summer walking the hay field in Mid Ohio shooting ground hogs with my .270 and got really good at sitting quick, wrapping in the sling, settle and squeeze.
That fall I jumped a group of elk while tracking in the snow, I caught movement below me, and before I knew it I was sitting, slung, safety off, and a cow was looking at me while quartering away. 150 yards, boom, dead cow elk. It was sort of automatic.
Now I do as others have said. I lean into my MR pack for a really nice stable field bench... haha