A barrel vent is a port(s) in the barrel wall, commonly near the muzzle, through which gases formed during propellant burn escape prior to the projectile(s) exiting the barrel.

related:

• muzzle break

Footnotes / Sources:

“Glossary ⇒.” Sporting Arms and Ammunition Manufacturers’ Institute. Ed. SAAMI’s Technical Committee. Web. 15-18 Jan. 2011.