spotting pistol

spotting pistol

[′späd·iŋ ‚pis·təl]
(ordnance)
A short automatic or semiautomatic firearm mounted coaxially on a larger-caliber gun, designed to conserve and to increase first-round hit probability of the ammunition used in the larger weapon; it employs a magazine and fires a spotter-tracer projectile which is ballistically matched with the trajectory of the projectile of the gun on which it is mounted.