sear spring

from The Collaborative International Dictionary of English v.0.48

Sear \Sear\, n. [F. serre a grasp, pressing, fr. L. sera. See
   {Serry}.]
   The catch in a gunlock by which the hammer is held cocked or
   half cocked.
   [1913 Webster]

   {Sear spring}, the spring which causes the sear to catch in
      the notches by which the hammer is held.
      [1913 Webster]