I enjoyed your post, Adamantium. Your Howa 6.5x55 experience echoes my own. My Howa 1500 shoots okay with Hornady 140 gr SP and Hornady 129 gr SP. It doesn't seem to like lighter bullets at all. 120 gr bullets (Nosler BT, Sierra MK, Speer SP) sometimes shoot okay, but attempts with lighter bullets Speer 90 gr TNT, Nosler 100 gr BT were all over the paper or even off the 8 1/2 x 11 target at 100 yards off a sandbag rest. I originally hoped to load the Nosler 120 gr Ballistic Tip at around 2800 fps for deer. Sadly, consistent accuracy was not achievable with this bullet, and neither was 2800 fps. Best accuracy with my Howa seems to be achieved at low velocities. It seems to shoot best at around 2400 fps with the 129 gr Hornady and around 2300 fps with the 140 gr Hornady. I really don't care for packing a 150 yard cartridge, so I probably won't hunt with it. Also can't say 5 shot groups of 2 inches or so isn't exactly confidence inspiring, either. The Howa action is very smooth, and it has been very reliable feeding and firing and extracting. It just isn't very accurate or high performing. For all that, I still can't bring myself to get rid of it. I still dream of finding that load that will be decently fast and accurate. And it is pleasant to shoot off the bench. (I even ordered a laminated Boyd's stock for it and glass bedded the recoil lug. Something I have not had to do before or since with other rifles in other chamberings.)
So if you do hit on a "magic load", be sure to post it, as I would be willing to try almost anything just to see if a consistent load could be put together.