Stopping Power ( or whatever terminology regarding the probability of shootee falling down in a timely manner ) with fmj from rifles will involve tumbling of the bullet, ideally tumbling AND fragmenting . Put another way the goal is for the bullet to be unstable in flesh . For the bullet to reach the flesh, it must be stable enough in air to be reasonably accurate.

Generally the factors are length of bullet and rate of twist . To oversimplify the goal would be a bullet just barely stable enough to reach the target. This can be aproached from either direction. Such as in the early '60s with 55gr in 1 :14 , and recently with 77gr in 1: 7 .

I've done a little with 62gr in 1:12 , and intent eventually to work with 69gr in 1: 9 .