I've ran three types of ammo thru it. The first group measured .6" and after that it went to hell. Hasn't shot under 2 inches after that.
The problem might be that the barrel isn't as clean as you think. The WSMs will foul and the higher pressures will bake the fouling. I have a T3 in 300 WSM and spent hours scrubbing and patching the bore when working on my loads, and it would still have patches with fouling. Finally got tired of all the scrubbing, and bought Wipe-Out - a foaming bore cleaner. I spray it in the bore, leave it in overnight - doesn't contain ammonia, so it is safe - and patch it out the next day. I have been using Wipe-Out for the last 5 years and highly recommend it.
Your barrel will eventually break-in, and there will be considerably less fouling. Mine shoots extremely well from a clean barrel.