If your only shooting deer at 300yd, why in the world would you waste the time and money on a STW??? Get a 243.
The STW is a long range gun. 500yds is where it starts to perform good. Try the 162gr A-max. Excellent BC and excellent performance on deer and elk. My daughter just shot her first elk at 823yds with an STW and A-max and quartering away, the bullet was lodged against the hide on the opposite side. PERFECT mushroom. This is the first bullet to be recovered from this gun. Most are in-outs.