I have only one thought on this, I wonder what Randy Garrett would say about this, his tests with a 45-70 seem to show that penetration slows down at higher velocities, 1400fps as opposed to 2000fps.

His comment was "The faster the velocity the faster they slow down." when penetrating media. I am only commenting out of interest in this, not to create acrimony among shooters.

I do not own a 45-70 or 600 Overkill.