I have seen many debates carried out in the rifle and calibre pages, but really, the hole in my pipe is bigger than the hole in your pipe is getting lengthy and sometimes tiresome.
External ballistics is the viewpoint that many people come from particularly when it is anything to downing the 6.5. Lets try to leave calibre out of it at the moment and I am sure some will attempt to go down that route as quickly as possible.
Lets try to debate what actually puts a deer on its arse, other than a spine shot .
I will start the ball rolling by saying that irrespective of the calibre used, the bullet must deform in such a reliable way and maintain its ability to penetrate at least as far as the vital organs with the diameter of the projectile reaching about 2 to 3 times its original diameter, causing a high level of cavitation on its way through. The underlying issue is to have a calibre large enough to actually go through the animal beyond the vitals, but this needs to be coupled with the correct selection of bullet head.