Abstract: An apparatus for propelling an elongated device including a sharp object such as a hypodermic syringe and needle or the like into biological tissue, the apparatus including an inner sleeve for carrying the hypodermic syringe or the like and an outer sleeve in which the inner sleeve reciprocates, biasing means being provided for biasing the inner sleeve relative to the outer sleeve. The locking sleeve receives therein the outer sleeve and through at least one recess and ball bearing arrangement, the inner sleeve can be locked relative to the outer sleeve. When the locking sleeve is forced against biological tissue, the ball bearing or bearings release the locked relationship between the inner and outer sleeves and propel the needle of the hypodermic syringe or the like into the biological tissue. An improvement in assembly is provided by the present invention such that the ball bearings can be placed in position without the boring of a ball bearing loading aperture in the locking sleeve.