in aerosol use with a very few exceptions. Its use is also banned in many countries under United Nations treaties. On December 31, 2008, the use of CFCs for medical inhalers were banned. Later users modified these old guns to be powered by regulated CO2 canisters or nitrogen/high pressure air bottles to increase power and consistency. However, these guns have largely been superseded by the newer and more versatile AEGs, or automatic electric guns. One of the reasons for this is because the most commonly available propellant, R-12, is costly. Additionally, at high flow rates, liquid propellants tend to cool down, eventually freezing. As cooldown progresses, the rate of fire gradually decreases until the gun ceases operation. The user would then be forced to wait for the propellant to warm up again. CO2 is not affected as badly by this tendency, and nitrogen/high pressure air is immune to it. Furthermore, if liquid propellant is introduced into the gun's mechanism, rubber parts can freeze and eventually damage the gun. However, it is unlikely for this to occur since once the gas is released from the containing cylinder it instantly turns back into its gaseous state, and expands rapidly. It is doubtful whether the retained pressure behind the BB before it begins to accelerate down the barrel is enough to keep the gas in a liquid form. Also, any gun that is expected to be exposed to the intense cold of de-pressurizing gas should have materials that can handle it. Gas power tends to be used in airsoft pistols where size constraints make electric-powered mechanisms impractical. Other instances where gas is favored are where adjustable velocities are required or where a blowback feature is desired. A blowback feature is a mechanism which cycles a slide or bolt to better simulate a real firearm's operation. Because of the mechanical complexities involved with distributing and regulating gas, these guns have largely given way to electric guns for less specialized applications, however, they still remain favorable amongst most airsofters. They are not just limited to pistols; submachine guns, sniper rifles and assault rifles commonly use gas mechanisms. Whilst the submachine gun replicas typically feature a blowback mechanism similar to the pistol replicas, sniper rifle replicas usually omit the blowback mechanism in favor of reduced recoil and increased muzzle velocity. Along with using gas to power guns, it is also applied for use in replica grenades.