Abstract: Injector administering subcutaneous injections without a needle and with a one-shot cap. The injector (11) includes an injection head (16), a chamber (27) to contain medicament which cooperates with a piston (28), an outlet aperture (15) for the medicament, means (30) to convey the medicament into the containing chamber (27), means to compress means which expel the medicament and a cocking and trigger assembly to actuate the expulsion means. The injector may also include means to measure the quantity of medicament charged and possibly means to limit the charging. In cooperation with the injector (11) is a frontal replaceable one-shot cap (10) with a removable stopper (12) which is broken in a pre-set manner. An axially movable striker (26) is also included and has a first breakage position and a second injection position defined by the replaceable one-shot cap (10).