The Collaborative International Dictionary of English v.0.48:

gunstock \gun"stock`\ (g[u^]n"st[o^]k`), n.
   The stock or wood to which the barrel of a hand gun is
   fastened.
   [1913 Webster]

WordNet (r) 3.0 (2006):

gunstock
    n 1: the handle of a handgun or the butt end of a rifle or
         shotgun or part of the support of a machine gun or
         artillery gun; "the rifle had been fitted with a special
         stock" [syn: stock, gunstock]