there are green gas guns red gas guns propane guns and co2 guns. they cant be mixed
yes
C02 is the gas that fires the BB. It's made up of Carbon Dioxide and comes in a cylinder that you place in the gun.
Most stores will carry an M14 replica either walk in or online. I would reccommend (See the Link Below)i deffinetly would go with the first link, a really good m14 on gi is the cm032 or cm032a
Walmart