Mix the vinegar in the wash once its filled than throw some baking soda in there. The reaction that the two have will relieve some of the smell and mildew. My wife uses this combo on my short stains and works great. And no I am not trying to blow your washer up.
I would choose 6.8, because it is a beast of a round. Hogs are no problem and anything 100yrds plus I could feel confident with it. I don't know too much about the 300 round, but I know for a fact I haven't meet anyone who hunts with it....yet