B.B. has written a very interesting post about a working replica of an outside lock air rife. This type of air rifle was made between 1600 and the mid 1700s. It is called an outside lock because the firing mechanism is literally on the outside of the action.
Instead of knocking open a value with a hammer and instantly releasing gas when the trigger is pulled, a cam is used to time the release of the gas. It creates a very effective system, although to take advantage of the slow release of gas a 32 3/4″ barrel is needed.
I highly recommend this article.