All that aside I would bet Tom is right the twist rate was too slow to stabilize that long RN 220 bullet and it began to tumble on impact.
If the rifle is 1:10 it's got more than enough twist. RN bullets are shorter than spitzers and dont require such a fast twist because they're short for their weight. If the bullet isn't stabilized enough, it's going to be tumbling through the air, not through flesh. AFAIK soft point bullets dont typically tumble in flesh, they're meant to open up and travel straight through. If the bullet tumbles, I'd assume it'd gain penetration, if it's not shedding weight since the large frontal area of an expanded soft point typically causes more drag than a yawed bullet.
According to the OP the bullet only lost about 40 grains of mass, which actually isn't too bad for a lead core bullet at .300 Win mag velocities.
The length of the 220 grain RN is 1.283''
The length of a 190 grain Nosler ABLR is 1.460
The length of a 150 grain Nosler AB spitzer is 1.230''
The 220 grain bullet is barely longer than a 150 grain spitzer, and due to its significantly higher mass and more forward center of gravity, it may actually have more stability than a 150 grain spitzer bullet, in the same barrel.
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