Abstract: A tip cap assembly adapted to close a fluid passageway extending through a tip of a container of a medical injection system, the tip cap assembly including: a rigid outer cap including an inner skirt extending proximally and having an outer threaded surface, an elastomeric inner cap arranged within the outer cap, the inner cap having a protrusion extending proximally and adapted to close the fluid passageway, wherein the outer cap further includes an outer skirt extending proximally and around the inner skirt so as to define an annular groove between the inner skirt and the outer skirt, and that the outer skirt includes a proximal member having a projection extending inwardly or a recess depressed outwardly.