decreasing pressure signs?

I had an interesting experience recently involving high pressure loads in a .308 Win. I've had unbelievable accuracy with very heavy bullets (220g HPBT MK) but they seem pretty unstable. one 10 shot group @ 0.5" and several 5 shot groups @ 0.5" @ 200yds. But if there is ANY wind those groups are out the window and I see key holed bullets @ 200 yds. I guess that a 1:12 twist is just not quite enough for that bullet @ these velocities.
So, in an attempt to stabilize them, I started increasing the loads in 0.5g and 0.25g increments. Increasing the velovity thus had the desired effect. greater velocity through the same rifling produces greater RPM applied to the bullet. I've gotten them pretty stable now. No chronograph, but they shoot great.
Here is where it gets interesting. My original load was 39.5g of Reloader 15 under a 220g Sierra HPBT MK. I increased this, as describeed above, ultimately to 42.5g of Reloader 15. Moving in .25g increments, pressure signs(flattened/flowing primers, head expansion, ejector marks on the case head etc. ) peaked @ 41.5g, decreasing all the way to the max load I settled on.
One thought I had was that the barrel grew warmer with the hotter loads and resulted in a slightly larger bore diameter. The following day I shot 60rds of that load (42.5g RL15/220g HPBT MK) watching for any sign of high pressure. Nothing. Anyone have any thoughts on this?