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A touch hole, also known as a cannon vent, is a small hole at the rear (breech) portion of the gun barrel of a muzzleloader small arms or cannon. The hole provides external access of an fire making spark into the breech chamber of the barrel (where the combustion of the propellant occurs), either with a slow match (matchlock), a linstock or a flash pan ignited by some type of pyrite- (wheellock) or flint-based gunlock (snaplock, snaphaunce, and flintlock), which will initiate the combustion of the main gunpowder explosive charge.Vauban and the French military under Louis XIV, Jean-Denis G. G. Lepage, p.38 Without a touch hole, it would be nearly impossible to ignite the powder because the only otherwise access into the barrel is from the front via the muzzle, which is by the projectile.
In the later caplock firearms, the ignition sparks are generated by a shock-sensitive percussion cap placed over a conical "nipple", which has a hollow conduit that leads into the barrel's back chamber known as the flash channel. When shooting, the percussion cap is struck by a spring-cocked hammer, causing the inside to generate primary sparks, and the flash channel serves the same function as the touch hole to transfer the sparks from the percussion cap into the barrel to ignite the propellant.
In modern breechloading firearms, the propellant charge is packaged inside a cartridge, and ignition is generated by a modified percussion cap (known as a primer) seated inside a cavity at the back end ("head") of the cartridge case, which is struck by a firing pin during shooting. Between the primer pocket and case chamber are one or more apertures known as the flash holes, which serves functionally as a touch hole inside the cartridge to transfer the ignition sparks.
In artillery, priming powder, a fuse, squib, or friction igniter is inserted into the touch hole to ensure ignition of the charge. The ignition might be achieved via striking or electrically.
Count Friedrich Wilhelm von Bismarck, in his Lectures on the Tactics of Cavalry, recommended that every cavalry soldier carry the equipment needed to spike guns if an encounter with enemy artillery was expected. If a cannon were in danger of being captured by the enemy, its crew would spike the gun to prevent it from being used against them. Captured guns would be spiked if they could not be hauled away and the gun's recapture seemed unlikely.
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