Hi Tony,
Just to muddy the waters a bit more for you, it should really be referred to as a 'Caliver'. The Caliver was a shorter version of the Arquebus - although both were matchlock. The standard matchlock of the Civil War period came in a variety of bores, and this caused problems for the musketeers on the battlefields - who often found that they had to deform the shape of musket balls to make them fit the barrel. A shorter version of the Arquebus was introduced with a standard bore of barrel, and this was known as the Caliver . The word Caliver was a corruption of the Italian word for internal measurement - which was Calibre. And that is basically how the word Calibre or Caliber came to mean the size of the bullet for any particular weapon.
The matchlock was replaced by the wheellock - a system of igniting the powder which had many advantages over the matchlock, but was rather complicated and expensive to make. The (VERY) basic principle of how it works is when the trigger is squeezed, a wheel spins and causes sparks, which in turn ignites the powder in the pan. After that came the Snaphaunce and Doglock. The Doglock had no internal half-cock position, so the hammer was held back by a catch - the 'dog' - which engaged with the hammer as it was drawn to the rear. The gun could then be safely loaded and the hammer fully drawn to the rear and ready to fire.
The next stage in the evolution of these weapons was the Flintlock, with this variation having an internal half-cock position, As the hammer was drawn to the rear, it was held at half-cock and rendered safe to load. This next bit is off topic, but serves to illustrate the use of half-cock as a safety feature.
Even some relatively modern weapons used the half-cock as a safety device rather than a proper safety catch. The Maxim-type guns had no safety as such, and during WW1 the guns were kept at readiness by having them at half-cock. Maxim type guns had to be cocked twice to load them, the first pull of the crank merely placed the extractor over the base of the round in the belt when the action was cycled. This was the half-cock position.
On the second pull of the crank, the cartridge was withdrawn from the feed belt and taken to the rear. As the lock returned to battery, the round was fed into the breech, the extractors moved up and grabbed the base of the next round, and the gun was ready to fire. Unlike other machine guns, the Maxi guns - of which the Vickers gun was a modified Maxim - the gun fired from a closed bolt. In other words, there was ALWAYS a round in the breech unless the gun was at half-cock. This was why Maxim guns were water cooled - it was to stop the round cooking off in a hot barrel.
Cheers,
Steve