Hi Seekay
early gun started out as muzzle loaders using black powder (gunpowder) a mixture of sulphur, charcoal and potassium nitrate (salt peter) as the propellant. Ignition was by a slow burning match used to ignite a small amount in a pan mounted on the side of the barrel by the breach which has a vent or touch hole, known as a matchlock.

Ignition improved by a Wheelock which spun a hardened steel wheel which was serrated against a piece of iron pyrite which sent a shower of sparks into the pan. This was a very complex mechanism given the technology of the era, knights in armour etc.

A simplification came with the Snaphaunce lock, the doglock and then the flintlock all of which used a flint hitting a steel plate to produce sparks that ignite the powder in the pan. Then the main charge.

the next improvement was the use of a mercury fulminate cap which when hit by the hammer cause a small explosion. These where placed on a nipple to the side or behind the breach and the flame passed through a small channel to ignite the powder.

Improvement in the caps allowed them to be mounted in the base of a cartridge case which in turn allowed the gun to be loaded from the breach with a self contained ignition system, main charge and pellets rather than muzzle loading.

The early version used a pin protruding at right-angled to the cartridge case just by the base. Small slots in the breach normally at the top meant that when the gun was loaded the pins stuck out of the top of the closed breach and any impact could set the cartridge off.

Then came the mounting of the cap or primer into the base of the cartridge and the need for a firing pin to strike the base of the cartridge becoming part of the gun's mechanism.

The change from black powder came via Mr Nobel's experiments to provide a more powerful, non smoky propellant; he came up with the nitro powders, and is the same guy who having made a packet out of arms sales introduced the Nobel peace prize. Good PR.

Old guns that are proofed for BP must not use nitro powders it can prove lethal to the user and any who stand near by unless they have been re proofed for Nitro powders. BP guns can use BP alternatives but why bother they were made for BP.

Nitro proofed guns can use BP or the alternatives - it a form of down rating the power.

Hope this is of help