instantaneous initiation of the primer thus by passing the intended functioning of the delay detonator does not occur portion 14 may be of the same composition as charge 1a a liner 17 in perforation 3 39 34 and or a thick wall 17a on coupling element 4 39 34 may be used to assure the delay functioning also in the primer unit shown in fig2 coupling element 4 39 fig2 a is a double walled shell 9 made for example of metal or plastic and having a bore 6 39 a shock sensitive high velocity detonating explosive powder e g petn or rdx 10 is loaded into the annular space between the walls of shell 9 and an annular plug 11 seals the end of shell 9 closed wall 17b is thin enough to allow initiation of explosive powder 10 by ledc 13 double walled shell 9 may also be of molded plastic in the embodiment shown in fig3 coupling element 4 34 fig3 a comprises a shock sensitive high velocity detonating explosive powder e g petn rdx and nitromannite 10 34 enclosed within an elongatable elastomeric package such as a balloon 16 the balloon is wrapped spirally around a thin walled tubular support body e g a plastic straw 12 which has an inner diameter chosen to closely match the outer diameter of an ledc 13 to be threaded through the primer straw 12 serves to confer the tubular configuration on coupling.