wire 30 is adapted to proceed from the rest position depicted in phantom in which it is biased in the direction of the tube 20 by either its own resiliency or other means to a position in which the opposing wall 28 is contacted the deformation of wall 28 not being shown obviously the momentum of the wire 30 must be sufficient to deform the opposing wall 28 the extent necessary to crush the primer mixture lying immediately therebeneath which action actuates the tube primer fig5 and 6 illustrate different shapes which the tube may be caused to assume the indented segment of the tube 20 may be subjected to a mechanical swaging operation to cause it to assume the generally circular shape of fig5 the swaging operation must be performed before the primer mixture solidifies as will be explained hereinafter two advantages of the configuration of fig5 over that of fig3 are a reduction in diameter and an increase in the arcuate length of the sensitive area of the opposing wall the indented portion of the tube may also assume a shape as shown in fig6 in fig6 it will be noted that the apex of the anvil is somewhat pointed since.