I shoot 6.5x55 with 123 Fox Classic Hunter monometal bullets. I homeload them so they’re much firmer than the weak factory offerings, running at just around 2,900 fps.
They’re very good on all deer up to sika which I manage in a park.
I also have a .300 WinMag barrel which I use in Scotland on the red deer. They will run on a little with the 6.5 but basically drop to the spot with the WinMag.
Basically, it’s physics. The homeloaded 6.5 produces 2,200 ft/lbs at the muzzle. The homeloaded WinMag is 3,500 ft/lbs by virtue of a bigger bullet travelling just as fast. That’s over 50% more energy hitting the deer and the energy has to be dissipated into the body causing it to drop to the shot.
I love the 6.5, but when I’m faced with a deer that might cover 50/60 m of clearfell after the shot, I love the WinMag more.