THE OFTEN UNDERRATED AIR GUN
(Page 5 of 6)
May/June 1981
Excerpt by Jim Carmichel
C02 GUNS
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The mechanics of CO 2 guns are quite similar to those of pneumatic guns. The principal difference is that the CO 2 gas is pre-compressed rather than being compressed in the gun as needed. The exhaust valve, venting, breeching and firing mechanisms of CO 2 and pneumatic systems work in such similar fashion that on some models of each by the same manufacturer many parts are interchangeable. In other models of CO 2 guns, considerable individuality is apparent.
Though certain other gases could be used as a power source, carbon dioxide has the physical and chemical advantages of being nonflammable, noncorrosive, odorless, inexpensive and not inclined to react chemically.
The CO 2 in the familiar little seltzer bottle cylinders is compressed into liquid form. When a CO 2 cartridge is inserted into the gun and the steel punctured, a certain amount of the liquid CO 2 boils off in gas form and fills the gas reservoir. When a specific amount of gas has boiled off, the resulting vapor pressure keeps the remaining carbon dioxide liquid. This fortunate characteristic, as we shall see, is important to the practical operation of CO 2 guns.
When the trigger is pulled and the striker opens the exhaust valve, the gaseous CO 2 is vented off to propel the BB or pellet. The exhaust valve closes immediately, and CO 2 gas is again boiled off until the vapor pressure reaches equilibrium with the remaining liquid. The evaporating CO 2 has a refrigerant effect which noticeably cools the gun's chamber and barrel. This cooling effect can result in a slight, but steady, reduction of gas pressure if the gun is fired rapidly. Under normal conditions, however, the vapor pressure is constant and results in quite uniform velocities. The boiling-off period of CO 2 gas, by the way, is virtually instantaneous, occurring faster than one can pull the trigger.
The speed and ease of shooting CO 2 guns causes them to be fired more freely than other types of air guns, and this leads to the main objection: the expense of replacement gas cylinders. The number of shots per cylinder in most guns ranges from 30 to 80 or more. The last few shots from a nearly empty cylinder may have a noticeably lower velocity than normal. Some models feature a low-pressure release valve which automatically empties the cylinder when the CO 2 pressure falls below practical shooting levels.
CALIBERS
Though there have been air guns that fire bullets as large as .60 caliber and as small as No. 6 shotgun shot (.110 inch), there has been, since the turn of the century, a trend toward standard sizes of air gun calibers. At present the four principal calibers are rather loosely known as .17, which includes BB's and .177 pellets; .20, which is the standard Sheridan caliber; .22, which includes a large array of pellet-firing pistols and rifles, and .25-caliber pellet and dart guns.
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