Sectional area of the passage for gases divided by the length of the same passage in feet
The narrow opening leading from the upper side of the breech to the after end of the chamber, used for igniting the powder charge to fire the piece Earlier guns were primed with loose powder, and their vents are often surrounded by cup-shaped moldings to receive powder Later guns used priming tubes, and their vent openings are flush with the barrel Hot powder gases are highly erosive, and the vents of bronze guns were enlarged by repeated firing In later bronze guns, a cooper vent bushing, or vent piece, was screwed into a threaded socket When the vent became enlarged the bushing was replaced