Originally Posted by kstrick
I check the scale with the weight set every 6-8 throws. It appears to be very accurate.
Checking an electronic scale with check weights will only find gross errors, not the sort of problem you have. You have to check the whole process that leads to the quantity of powder you pour into the brass. The "only" way to do that is to weigh "every" charge on a good quality balance beam scale.
The reason for this is two fold. First, electronic scales have a deadband (plus or minus tolerance) built in or they can't work. The size of the deadband required to allow the scale to settle depends on the quality of the electronics being used to read out the strain gage bridge that is the heart of the scale.
My electronic scale always, every single time, gives me the right answer with check weights but it throws charges with an extreme spread of +/- 0.3g. The extreme spread in charge variation is what must be controlled. You have to weigh the charge that is thrown by the charge master to find thrown charge variation issues. Weighing check weights won't find the problem because they don't check the whole process including the dynamics of adding powder to the pan.
Which leads to the second cause which is the process for metering the charge feeds powder to the pan in clumps as the weight gets clost to the desired final weight. The weight increases in clumps as powder dumps out of the tube. It is almost impossible for the scale to have a long run extreme charge spread smaller than than approximately plus or minus the largest clump size. That's just how it is. You get charge spread you get velocity spread.
The two causes work together to create the extreme charge spread that happens with the ChargeMaster. If the final clump just crosses the threshold, the scale will stop the power trickler which is what the CM is and the charge will be on the low side of the deadband. If a clump falls on the scale that takes it to the upper end of the deadband, then the charge will be heavier. If the last clump was under weight but large enough to dynamically trip the comparator in the scale, the charge will be under weight.
Electronics are fine but the low cost electronics in powder scales aren't all that great so they have a bigger dead band than laboratory quality scales. Near as I can tell, electronic reloading powder scales are the cheapest thing that marketing thinks will sell the product. That's just how businesses are run. The most profitable businesses are those where they are really good at barely getting by.
The RCBS ChargeMaster is the best of the bunch near as I can tell, but it isn't nearly as accurate as trickling to weight on a balance beam. My balance beam isn't perfect, but checked with a borrowed very accurate laboratory scale (accuracy of +/- 0.02g) it demonstrated extreme variations under +/- 0.1g.
Try weighing every charge as you reload - measure a hundred thrown charges. You will see more variation than you would ever have thought. I certainly did. The variation is the largest with big stick powders like IMR7828 and almost non existant with powders like IMR2460 (that looks like tiny shiny black ball bearings) that flow almost like water.
There are lots of other things that can cause velocity variation. Primer hole flash, variations in neck tension, seating to just touch the lands so some bullets touch, some don't, variations in crimping if the bullet is crimped into the neck, variations on overall length, worn out barrel (throat shot out or huge chatter marks in the barrel which one would need a borescope to see), and so forth.
Your problem is probably not caused by just one thing. Contributors will have an effect that combines by the square roof of the sum ot the squares of their individual contributions. As you fix the causes, the variations will be less and less, but might still be dissapointingly large if the remaining cause is a more major contributor.