Factory ammunition is always loaded on the short side to ensure that they'll function in all, for the most part, production rifles. I load all of my rounds to an exact OAL. It only takes a hundredth of an inch or two in some cases to bugger up your accuracy. Like for testing the powder charges, everything about the rounds will be the same (OAL, case brand/length, primer) except for the charge weights. If you throw more than one variable into the mix, you'll never know which change in composition caused the good/bad results to appear. I was able to get good results with my 300 WSM using H414, Hy100V, and both IMR & H 4350's with the 180g AB so it looks like you're on the right track. When I went to the 200g AB, I got the best results with slower burning powders like IMR 7828 and H4831SC with 7828 being the most accurate in my rifle for that bullet.