All, I finally got chance to head back to the range, armed with many tips and tricks provided in this thread. Allowing the barrel to cool to the touch between each shot (with the help of a rifle kuhl fan, about 10 minutes between shots), I was able to get better accuracy! I tried Hornady 95 gr Vmax and 129 gr Interlock soft points, both printed some respectable groups at 100 yds, around 1.5" or better! Here is a pic of one of the representative groups (using Hornady Vmax bullets, squares are 1"):
And now, the next issue I was hoping to get help with...I am seeing incomplete case head separations in about 50% of my cases (tear in brass just above the case web). This is only the second firing on these cases, should I be trying to neck-size only to headspace on the shoulder, instead of the rim? Any pointers on how to accomplish this with a break-action? I will eventually perform a chamber cast, hopefully the chamber isn't oversized, or something like that!