ANY of the plastic-tip bullets tends to open very quickly, the reason they make such a big hole behind the entrance. This is is just part of the mechanics of those little polymer points: They decelerate very quickly on impact, opening up the bullet, often extremely.
If you are looking for a good meat bullet something with a conventional lead soft-point would be a closer match. In my experience with the .260 and 6.5x55 about any soft-point from 120-140 grains will work OK, but the heavier and slower they are the less meat gets messsed up. That's the reason the classic load of a 140 at 2700 or so is such a favorite among many hunters in either the .260 or 6.5x55. Believe me, it shoors pretty darn flat,with hardly any difference between it and and 120 going 200 fps faster.
