Let me restore your confidence. I have a Remington 700 7mm mag and I have never shot a dear that walked more then 10 yards.
The set up I use is a burger 168 grain hunting round backed by 64.4 grains of H-1000 On winchester brass. My scope is a 9x25x56 and even at 700 yards it will shoot a 3 inch group with a factory barrel.
I am guessing its your ammo. The burger will hit with about 900 lbs of force. It has a boat tail design and will fly flatter longer and will hedge out the 300 win mag.