Shooting at deer I have never seen anything big enough that I would risk wounding it to a poor judgement shot...myself I feel comfortable to 300 yards......but like DD said.....is it 300 yards?? If you can point it with a rangefinder then the battle is half won.
I shoot a Sako 25.06, I am sure ina better mans hand, on a calm day, with all perfect conditions, it will go 500 yards easily.....but you will never see me try a shot like that...even with a rangefinder.