How do you know whether a "gun fits"

Whenever I have been hunting up to this point a friend hand me one of his unused rifles and I shoot with that, take a few pratice shots to get the feel of it and then off we go.

Sounds wrong now that i write it down but all my kills have been one shot one kill.

When a gun fits will I know? i am a bigger guy, 6'4" so would one of these guns that I have thought about be better then the other for a guy my size?