Miller, here's the commonly used definition of Percussion System.
Percussion Lock - Based on a discovery by the Rev. Alexander Forsyth, patented in 1807, that a blow to fulminate of mercury will detonate it, through several designs to utilize the concept with limited success, culminating in the adoption of the copper priming cap. This small, cup-shaped cap, containing a bit of fulminate, after the hammer is cocked, is placed upside-down on the tubular-conical nipple. To fire the gun, one pulls the trigger, releasing the spring-loaded hammer which falls on the head of the percussion cap, detonating the fulminate, sending fire through the hole in the nipple to the main charge inside the breech (having been loaded from the muzzle) touching it off and discharging the weapon. The percussion system superseded the flintlock system generally around 1820 because it was more reliable in the wind and the rain, quicker to load, of faster lock time and because it was cheaper to manufacture. Photo
To find photos of the Percussion System , go to this extract from the Hallowell website and click on the Photo icon, or look here.
http://www.hallowellco.com/percussion_lock.htm 
