Greenhouse gases are the cause of Greenhouse effect.The greenhouse gases which include CO2, Methane etc. trap the heat in the Earth's atmosphere and don't allow it to escape.
The weapon is fitted with a Leupold Mark 4. The rifle is also similar in appearance to the TAC-338 rifle that Kyle used in real life. McMillan TAC-338A sniper rifle with Harris bipod, Leupold Mark 4 scope, and AN/PVS-22 Universal Night Sight - .338 Lapua Magnum. Kyle taking aim with the rifle. Right side of the rifle.
The Unertl sniper scopes used by the U.S. Marine Corps house the optics in steel tubes that are mounted to a bracket on the top of the rifle. They weigh 2 lbs, 3 ounces (~1 kg) and are 10 inches (~25 cm) long. They are fixed, 10-power scopes with a 32mm objective lens.