 Complete the following phrase:  Modern primers are basically improved percussion caps with shock-sensitive chemicals e.g. lead styphnate enclosed in a small button-shaped capsule. In the early paper cartridges, invented not long after the percussion cap, the primer was located deep inside the cartridge just behind the bullet, requiring a very thin and elongated firing pin to pierce the paper casing. Such guns were known as needle guns, the most famous of which was decisive in the Prussian victory over the Austrians at 