Thanks for the advice. Yes, my barrel is loose in that area, and I had been pondering whether to try something like that; I'm glad to hear it's an accepted approach and seems to be helpful.

I tried 8 different reloads today (I remember that I had some flat base bullets, though in .311), so I did varying combos of .311 or .312 bullets, with differing load of IMR 4064. I used the one you suggested and it worked pretty well. I think I'm pretty close to getting a preferred load for this rifle. I just need some slight tweaking at this point.