The wheel lock
firing mechanism was one of the most reliable and consistent methods of firing a gunpowder weapon prior to the invention of the
percussion cap. Its drawback was that it was expensive to manufacture, complex in operation, and not as resistant to poor handling and harsh environments as the
flintlock.
Its name came from the fact that it actually had a wheel in the mechanism. The wheel was mounted on a spring-loaded ratcheting shaft, and prior to use, had to be wound like a clock. This was done with a separate t-shaped tool carried by the operator. That was the first problem. If you lost your tool, you were seriously screwed, as you could no longer wind your gun.
Once the mechanism was wound, you loaded and charged the weapon in a manner similar to any other muzzle-loader. When the trigger was pulled, the wheel would spin against a piece of flint that was held in constant pressure against it by another spring-loaded lever. The resulting stream of sparks lasted for over a second, and was therefore much more likely to ignite the powder than the single set of sparks created by a flintlock.
The cost and complexity of the wheel lock was balanced by its consistency of operation, which is why it is mostly found in sidearms for rich officers. That is why most wheel lock weapons are pistols, and those that were made were usually luxury items for nobles and generals. Even the most mundane wheel lock weapons had a high degree of craftsmanship and decoration, as they were precision instruments compared to any of their contemporaries.