A stopper (108, 200, 1600, 1900) for use in a cartridge or syringe of a medical device, the stopper configured to be disposed within a container closure system (114, 290), the stopper (108, 200, 1600, 1900) comprising: a shell (201, 1601, 1901) comprising a closed end (203, 1603, 1903) and an open end (202, 1602, 1902) with sidewalls extending between the closed end (203, 1603, 1903) and the open end (202, 1602, 1902) along a longitudinal axis of the shell (201, 1601, 1901), the open end (202, 1602, 1902) defining a cavity (606, 1906) and the sidewalls defining an exterior surface sized and shaped to fit inside the container closure system; and an insert (1610, 1910) configured to be inserted into the cavity (606, 1906), the insert (1601, 1910) sized and shaped to receive a force from a plunger rod and distribute the force to the shell (201, 1601, 1901) in order to advance the shell (201, 1601, 1901) into the container closure system, wherein the closed end (203, 1603, 1903) of the shell in the cavity defines a convex surface configured to be contacted and deflected by the insert (1610, 1910) upon insertion into the cavity (606, 1906), and wherein the closed end (203, 1603, 1903) of the shell (201, 1601, 1901) is made of an elastic and/or plastic deformable material and characterised in that the cavity comprises at least one venting channel (1820, 1920) extending from the closed end (203, 1603, 1903) of the shell towards the open end (202, 1602, 1902) of the shell (201, 1601, 1901), the at least one venting channel (1820, 1920) being sized and positioned to enable air and/or fluid in the cavity (606, 1906) to be expelled though the at least one venting channel (1820, 1920) during insertion of the insert (1610, 1910) into the cavity (606, 1906). a shell (201, 1601, 1901) comprising a closed end (203, 1603, 1903) and an open end (202, 1602, 1902) with sidewalls extending between the closed end (203, 1603, 1903) and the open end (202, 1602, 1902) along a longitudinal axis of the shell (201, 1601, 1901), the open end (202, 1602, 1902) defining a cavity (606, 1906) and the sidewalls defining an exterior surface sized and shaped to fit inside the container closure system; and an insert (1610, 1910) configured to be inserted into the cavity (606, 1906), the insert (1601, 1910) sized and shaped to receive a force from a plunger rod and distribute the force to the shell (201, 1601, 1901) in order to advance the shell (201, 1601, 1901) into the container closure system, wherein the closed end (203, 1603, 1903) of the shell in the cavity defines a convex surface configured to be contacted and deflected by the insert (1610, 1910) upon insertion into the cavity (606, 1906), and wherein the closed end (203, 1603, 1903) of the shell (201, 1601, 1901) is made of an elastic and/or plastic deformable material and characterised in that the cavity comprises at least one venting channel (1820, 1920) extending from the closed end (203, 1603, 1903) of the shell towards the open end (202, 1602, 1902) of the shell (201, 1601, 1901), the at least one venting channel (1820, 1920) being sized and positioned to enable air and/or fluid in the cavity (606, 1906) to be expelled though the at least one venting channel (1820, 1920) during insertion of the insert (1610, 1910) into the cavity (606, 1906).