Monkeywerench: I get 9.7MOA for 600 with that load at 2k ft altitude at 60F. Keep in mind your zero not being 100% spot on will affect you at long range. You're ~1MOA off so I suspect your zero isn't perfect and / or your calculator doesn't like you using both barometric pressure and elevation. Some applications set that as an either/or.
If your zero is off at 100yrds by .5" then you'll see exactly what you're seeing now at 600yrds, within the cone of uncertainty formed by group size, shooter error and shot-to-shot MV variation so that's where my money is at. Everyone thinks their zero is dead nuts but very few people actually do get it dead nuts.
I've run several simulations and the above seems like your best answer Monkey. Feel free to PM me if you want to get into the weeds troubleshooting. I'll help you out.
