GregW,
You are correct about the 75 gr. Sierra HP being sturdy in the 6x45 cartridge. The lower velocity of the 6x45 makes the 75 grainer behave with a more controlled expansion.
Several years ago I had a Remington 700 heavy barrel in a 6mm Rem. that I used for groundhogs, and I used the 75 gr. Sierra HPs. I stocked up on some bullets. They were pure death on groundhogs and sometimes put some impressive wounds in them. When I got the 6x45, I decided to give the Sierras a try. What I found was this bullet in the 6x45 acts differently than from the faster 6mm Rem. cartridge.
Most of the time the exit wound is about the size of a quarter, give or take a little. It penetrates well but doesn't blow gaping holes in the fur. In my opinion, it's a great predator hunting cartridge and I like it better than the 223. For one thing, the frontal diameter of the bullet is greater and it packs a wallop, especially on coyotes with my limited experience hunting them.
In my experience, a 55 gr. soft point in a 223 does far more damage to bobcats, while the 75 Sierra HP performs in a better way, generally. I had one bad wound on a bobcat due to the angle where the bullet hit the shoulder.
Venatic,
Mike Milli told me he has a friend who uses the 6x45 quite a lot for deer and has taken several with it over the years. You are also correct that it is relatively popular in Africa.
The 6x45 is probably what the military should have developed instead of the 223.
Here are some 6x45 kills, all with the Sierra 75 gr. HPs.