bore sight

Short definition from Wiktionary

Noun

1. 1
   An optical device, mounted on or near the end of a firearm's barrel, used to align the sights with the bore.
2. 2
   A dummy cartridge containing a small battery-operated laser, used to align the sights.

Verb

1. 1
   To align the sights and aiming point of a firearm using a bore sight optical tool.