A rotary filling apparatus includes a rotatable drum body having an apex and a base, an outer circumferential wall which defines a drum periphery, and lateral walls which extend from the drum periphery into the drum body to define a plurality of spaced drum chambers. The lateral walls have aligned keyhole slots, positioned away from the drum periphery, and a vessel is slidably positioned in each drum chamber. A key, having a longitudinal axis and a cross-sectional area which increases from a base portion to a head portion, is positioned in the keyhole slots, so that upon longitudinal displacement of the key the vessel is caused to ascend or descend in the chamber, thereby decreasing or increasing the volume of the chamber. A cover plate is positioned adjacent the drum periphery, between the drum apex and base, for retaining material in drum chambers during rotation of the drum.