Sealant dispensing cartridge assemblies including radially expandable collets are disclosed. A plunger head assembly includes a ram head attached to a plunger rod. As the plunger rod is advanced into the sealant cartridge, the ram head presses against a rear side of a movable plunger against an opposing force created by a sealant contained in the cartridge on a front side of the plunger. As the plunger rod is further advanced, the collet contacts the ram head and radially expands against the inner sidewall of the plunger. The resulting radial force presses the sidewall of the plunger against the inner surface of the cartridge to thereby reduce or prevent unwanted leakage of the sealant around the plunger.