A feeder apparatus supplies a powder feedstock to a high or low pressure processing operation. The apparatus includes a metering plate defining an annular channel having a channel width. A measuring mechanism dispenses a measured volume of the feedstock into the annular channel. The measuring mechanism includes a pay-out device defining an outlet significantly smaller than the channel width. The measuring mechanism further includes a maximum fill level defining a maximum pressure head in the feedstock less than a retaining pressure defined by the internal friction of the feedstock. The restraining pressure prevents the feedstock from spreading outwardly in the annular channel and contacting sidewalls of the annular channel under the pressure head of the feedstock. The apparatus further includes a pick-up device for removing the feedstock from the annular channel. The pick-up device includes an inlet having a rectangular shape extending across the annular channel.