An ammunition manufacturing machine has a frame with a die receptacle, and a reciprocating ram having a free end movable toward and away from the die receptacle. The free end of the die receives a removable shell holder having a recess shaped to receive the base of a casing. The recess has a flat floor portion, and the shell holder is removable along a path lateral to the axis of the ram's motion. A retention element removably encompasses at least part of the shell holder and of the free end of the ram, so that the shell holder is secured by the retention element against lateral removal. The retention element may have a flat upper surface aligned with the floor of the shell holder recess, to provide a guide for sliding the case into the shell holder. The retention element may also have an internal O-ring that compressibly surrounds the end of the ram, to provide a friction fit that holds the retaining element on the ram.