THE OFTEN UNDERRATED AIR GUN
(Page 4 of 6)
May/June 1981
Excerpt by Jim Carmichel
The other disadvantage to accuracy is the very noticeable recoil, or bounce, generated by the powerful spring and piston motion. This is so troublesome to accuracy that top competition models feature special anti-recoil devices. The best of these work so well that the rifles and pistols seem not to move at all when they are fired. Some of these competition guns sell for over $600.
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PNEUMATIC GUNS
Pneumatic air guns are also known as pump-up guns because of the pumping operation necessary to accumulate and compress air in the "air tank" or reservoir. Modern pneumatic air guns made by Benjamin, Crosman, Daisy, Sheridan and Walther generally have compression chambers enclosed in the stock recess beneath the barrel and conveniently close to the breech.
Operation of the pneumatic system is simple. When the pump handle is pulled out, or extended, it withdraws a piston, or plunger, which works much like a bicycle tire pump. On the out stroke, air enters a cylinder that closely encloses the piston. On the in stroke, the piston forces the trapped air into the compression chamber through a valve which lets air in but not out.
With each stroke of the pump handle, an equal volume of air is pumped into the compression chamber. As the pressure within the chamber increases, it becomes progressively harder to pump in more air. Since heat is produced as air is compressed, the compression chamber becomes noticeably warm.
To fire the gun, another valve at the other end of the cylindrical compression chamber is opened, and the compressed air is vented into the gun's breech. It races up the barrel to drive the projectile. Velocity depends on both the pressure of the air and the volume of air let into the barrel.
This basically simple mechanism is complicated somewhat by the need for a sealed breeching system which must be opened for loading, and by a firing system which opens the exhaust valve. Since this valve is locked in a tightly closed position by high internal pressure, it requires considerable pressure to open it. It's comparable to opening a barn door into a high wind. This is why the pump-up firing mechanism actually knocks the valve open by means of a spring-loaded striker. When you cock a pneumatic gun (usually by opening the breech to load), you are actually withdrawing this rather heavy striker against spring pressure. The fall of the striker accounts for the solid thump you feel when you pull the trigger.
The accuracy potential of the pneumatic or pump-up system is excellent. Two top-rated tournament guns, Walther's LGR rifle and LP3 pistol, are of pneumatic operation. They differ from most pneumatic guns in that they are pumped up by a single stroke of the pump lever and feature virtually motionless firing (valve opening) mechanisms which result in recoilless operation. The Daisy pellet firing models 717 (.17 caliber) and 722 (.22 caliber) pneumatic-style target pistols feature singlestroke charging, as does the Daisy Model 850 pellet-BB rifle. Most pneumatics can be lightly pumped for plinking and target shooting, or they can be pumped to higher velocities for hunting or long-range shooting. This advantage is offset by the physical effort, noise and motion required to pump up the reservoir.
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