The old Lyman Ideal powder measures from the early 1900's had a neat little widget on them. It was nothing more than a tiny metal bar, or hammer, hinged on the side of the powder measure. In use, the reloader raised the bar once, allowed it to fall and hit the measure a sharp rap, and then he threw his charge. The mild impact and vibration settled the powder in the measure, giving a more compact and consistent charge. The old timers knew what they were doing, as even to this day good reloaders know to be consistent with operating the handle when they throw a charge.
It's this ability of most powders to settle with vibration that can cause a problem. If, like many reloaders, your powder measure is mounted to the bench along with the press, then each throw of the press handle can act as a 'powder knocker', settling the charge in the measure. This throws a random action into the job of measuring the powder charge, and inconsistent charges can play havoc with accurate ammunition.
If the measure is set to throw a maximum charge for a load, or is stocked with a very fast powder such as Bullseye, then the extra powder compaction can cause the thrown charge to be over the limit.
It's good practice to throw every charge of a set one after another, and then seat bullets in the case. The first few powder throws should be made into a clean container and dumped back into the reservoir. Never to be forgotten... check the occasional powder charge on a good quality scale to ascertain the measure is doing a consistent job and the settings have not drifted.
If the powder measure is left stocked with powder for some reason, it's always wise to begin the next loading session by throwing a few charges to be dumped back in.
My personal practice is to do so, but refrain from dumping the last sample thrown into the reservoir until after I am done charging the case set. I think even the act of pouring in a few grains of powder can settle the powder column, even if only a little.
Careful consistency is key!
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