I've seen a 2/10ths grain shift in charge weight averages with the same measuring chamber setting for a given lot of powder caused by:
* Extremes of powder column in the measure hopper; full hoppers put more pressure on the powder at its bottom and dispensed charges will be heavier. Improved baffling helps this.
* Humidity in the loading room; higher humidity means more moisture in the air and that adds to the charge weight as powder absorbs moisture.
* Speed and force the measure metering chamber is moved from dump to fill then cycled again as it stops at the mechanical limits. More shock to the measure makes more powder move into the metering chamber.
* The more solid the measure's mounted to what it's installed on, the less vibration it has when filling the metering chamber. If the bench the measure's mounted on is the same one the press is and the bench is not well fixed in place, press operation will effect how much powder settles to its bottom waiting to get put in the metering chamber; or how much powder's in the chamber and what's added with the charging port open letting more powder from the hopper settle in it.
All of which is not a big issue as a 2/10ths spread in charge weight seems good enough to set benchrest records and win matches through 300 yards. That said, some disregard this fact and almost cut powder granules apart to get weights to the 3rd decimal place. In spite of the best primers having a greater spread in heat and energy output than what 2/10ths grain of powder has.
* Extremes of powder column in the measure hopper; full hoppers put more pressure on the powder at its bottom and dispensed charges will be heavier. Improved baffling helps this.
* Humidity in the loading room; higher humidity means more moisture in the air and that adds to the charge weight as powder absorbs moisture.
* Speed and force the measure metering chamber is moved from dump to fill then cycled again as it stops at the mechanical limits. More shock to the measure makes more powder move into the metering chamber.
* The more solid the measure's mounted to what it's installed on, the less vibration it has when filling the metering chamber. If the bench the measure's mounted on is the same one the press is and the bench is not well fixed in place, press operation will effect how much powder settles to its bottom waiting to get put in the metering chamber; or how much powder's in the chamber and what's added with the charging port open letting more powder from the hopper settle in it.
All of which is not a big issue as a 2/10ths spread in charge weight seems good enough to set benchrest records and win matches through 300 yards. That said, some disregard this fact and almost cut powder granules apart to get weights to the 3rd decimal place. In spite of the best primers having a greater spread in heat and energy output than what 2/10ths grain of powder has.
