The vent pick is an artillerist's tool, used to prime the cannon before firing. Once the cartridge is rammed down the muzzle, the primer uses the vent pick to poke a hole through the cartridge inside the gun, then pours a small amount of powder into the vent to prime the gun. At the order "Give...fire!" the bombardier uses the linstock to bring a smoldering match to the priming powder, which ignites the powder inside the gun and the resulting explosion propels the shot or projectile (if there is one).
Vent picks must be made of brass, not steel, so as not to cause a spark when priming the gun. The resulting explosion would kill or maim everyone on the gun crew.