A plunger assembly comprising: a stopper (108, 300) comprising an elastomeric body including a distal end and a proximal end, said stopper including: an exterior surface; and an inner cavity, said inner cavity being a non-threaded cavity having smooth inner walls and comprising two adjacent frustoconical surfaces that connect to form a necked opening at a proximal end of the stopper, wherein the inner diameter of the necked opening has a larger diameter at the proximal end of the stopper and tapers to a smaller diameter within the inner cavity to form at least one engagement feature, wherein the inner cavity includes first and second frustoconical inner surfaces (308, 310), in which the first frustoconical inner surface is located proximal to the second frustoconical inner surface and distal to the necked opening, wherein the first and second frustoconical inner surfaces (308, 310) each include a relatively smaller termination end (316, 317) and a relatively larger termination end, and wherein the first and second frustoconical surfaces are orientated to join one another at the relatively larger termination ends; and a plunger rod having a threaded member at a plunger-engaging end, wherein said threaded member contacts said at least one engagement feature to support said plunger rod in an integrated, non-threaded engagement with said stopper, wherein a distal end portion of the plunger rod comprises the threaded member, and the threaded member includes a variable major diameter, and wherein at least one of the frustoconical inner surfaces (308, 310) is configured to engage the threaded member. a stopper (108, 300) comprising an elastomeric body including a distal end and a proximal end, said stopper including: an exterior surface; and an inner cavity, said inner cavity being a non-threaded cavity having smooth inner walls and comprising two adjacent frustoconical surfaces that connect to form a necked opening at a proximal end of the stopper, wherein the inner diameter of the necked opening has a larger diameter at the proximal end of the stopper and tapers to a smaller diameter within the inner cavity to form at least one engagement feature, an exterior surface; and an inner cavity, said inner cavity being a non-threaded cavity having smooth inner walls and comprising two adjacent frustoconical surfaces that connect to form a necked opening at a proximal end of the stopper, wherein the inner diameter of the necked opening has a larger diameter at the proximal end of the stopper and tapers to a smaller diameter within the inner cavity to form at least one engagement feature, wherein the inner cavity includes first and second frustoconical inner surfaces (308, 310), in which the first frustoconical inner surface is located proximal to the second frustoconical inner surface and distal to the necked opening, wherein the first and second frustoconical inner surfaces (308, 310) each include a relatively smaller termination end (316, 317) and a relatively larger termination end, and wherein the first and second frustoconical surfaces are orientated to join one another at the relatively larger termination ends; and a plunger rod having a threaded member at a plunger-engaging end, wherein said threaded member contacts said at least one engagement feature to support said plunger rod in an integrated, non-threaded engagement with said stopper, wherein a distal end portion of the plunger rod comprises the threaded member, and the threaded member includes a variable major diameter, and wherein at least one of the frustoconical inner surfaces (308, 310) is configured to engage the threaded member. wherein said threaded member contacts said at least one engagement feature to support said plunger rod in an integrated, non-threaded engagement with said stopper, wherein a distal end portion of the plunger rod comprises the threaded member, and the threaded member includes a variable major diameter, and wherein at least one of the frustoconical inner surfaces (308, 310) is configured to engage the threaded member.