Pneumatic
Pneumatic rifles are powered by compressed air, and there are a few different ways to get air compressed in the gun.
PCP (or Pre-Charged Pnuematic), which gives you the best of both worlds. Air is compressed in a scuba tank or a carbon fiber tank. A special hose with a pressure gauge attachments is used to connect the gun to the tank. PCPs have various configurations, and you can find both single shot and multi-shot versions of this air rifle.