Scale/check weight issue...

So I have a RCBS M500 mechanical scale. I also have a set of RCBS check weights. When I adjust the scale and set it at zero and check it against the check weights it never comes out right. Now usually I go by my chargemaster and check it against a smaller electronic scale when needed (they're usually within .1 of each other) and against the M500 if I feel really unsure, but the scale looks to be farther off. This has yet to be a safety concern as I tend to use powder charges under the maximum pressure. But for safety's sake if I do start using higher pressure charges, should I trust the scale or the check weights?

See if you have some grit in the pivot point on the scale. I did on my scale one time and it made the scale unreliable so I got another set of scales. when I was packing up the old scales I found the piece of grit so I cleaned the scales and tested them again and they were right on

0.1 grain is about 6 milligrams. If your check weight is a commercial 20 gram weight like mine, it should be within .1 milligram of actual mass. I would trust the check weights not any mechanical powder scale or electronic one. In the good old days when I took analytical chemistry, which was before electronic analytical balances, our mechanical balances were in special conditioned rooms, on concrete benches attached to a concrete wall that went down to the ground. The other way to do it was to put the balance on a rock or concrete slab that sat on a half inflated inner tube. We also calibrated each weight against a standard verified by the bureau of standards.

We did this because we needed to be within 0.1 milligrams of actual weight. That is equivalent to 0.0015 grains. My bottom line is that weighing to .1 grain is not all that hard. Just be careful about big drafts, and passing trucks and you will be fine.

"...within .1 of each other..." .1 of what? Ounces? Grams? Grains? 1 grain = 1/7000 of a pound. 1/10 of 1 grain is infinitesimally small.
Personally, if I had an electronic scale I'd be retiring the balance scale. However, what you sit the RCBS scale on matters a bit too. If it's not flat that can throw it off a little. Never had any kind of check weights myself in 40 some years of reloading. Don't over complicate the whole thing.