It's a long story, but the RCBS 1010 that I've been using for 20 yrs got inconsistant, and I didn't pick up on it. I weigh all powder charges on it. and had noticed for the last 2 years or so that powder charges were sitting at different levels in the necks of cases, particularly 7-08 loads. I just passed it off as different density as I dumped them out of the pan, but I also started getting some weird groups, like 4 shots together and one out that I didn't shoot. One night I moved a 49.5 gr load of H-414,that was dead on) over to my digital to check the digital and it showed 50.2 grs, so I re-zeroed the digital and set the pan back in the 1010. This time the beam pointer end bottomed and when I rolled the tenth's cylinder back to balance, it weighed 49.0 grains. I couldn't believe it, so I went back and forth with the same pan of powder and got weights all over the place on my 1010. I also dumped and re-weighed the 15 loads that were waiting for bullets, and half of them were way off.
After posting and talking to several people, the best bet was that the copper damping plate had developed some residual magnetism. Everything else was fine,,cleanliness, knife edges, agate bearings, etc) so as implausible as it sounded, I took the plate off and annealed it. It lost a 10th of a grain or so of weight when I glass-beaded the heat scale off, but after rebalancing, it has worked perfectly, with no more variation from load to load. Incidentally, the 250 gr test weight was still accurate before I annealed the plate. The scale was very inconsistent at light powder charge weights, but wasn't nearly as bad at 120 gr powder charge weights.
I don't have a better explanation, and haven't talked to RCBS, so am just passing this along.
Tom