Types of Rifle
Basically, there are two types of rifle:-
Pneumatic (where the power comes from a diver's bottle or other compressed air source). These can also be "pumped up"- side lever, underlever rifles- where power comes from a series of "pumps" at a spring compressing lever, but can then be supplied on tap like a pneumatic.
Spring / gas ram- break barrel- (where the power comes from compressing a spring or sealed gas cylinder).
There are also carbon dioxide powered rifles, but I don't think that these are suitable for hunting. Power is too inconsistent, depending on temperature, and CO2 will never be as efficient as air in a rifle. (CO2 is denser, so less gas passes through the transfer port while its open, and I think that CO2 cannisters are too small to provide a reasonable number of hunting power shots- carbon dioxide is fine in target pistols). Finally, the muzzle crack from CO2 is louder than that from a corresponding air gun.
BSA Super 10- a 10 shot pneumatic
Compressed air is supplied to the air bottle from a compressor, hand pump or diver's bottle to about 200 bars, and the gun will then fire around 200-500 full power shots (depending on gun, ammunition, range etc. etc.). As there is no spring to compress the air, these guns are virtually recoiless, and they can be repeaters (though you also get single shot pneumatics), but they are relatively expensive. (The Super 10 will set you back about £550 for gun and bottle alone).
These are similarly recoilless, and you can often do a variable number of pumps for each shot. Once pressure drops, you have to pump it up again, and the effect this has on your pulse rate does not help accuracy and "holding on target".
Spring / Gas Ram
Weirauch HW90 with Theoben gas ram
Spring and gas ram guns are virtually the same, except in their internal workings. Spring guns contain a spring (you don't say!) which is compressed by the action of breaking the gun's barrel at the pivot point (usually just before the end of the fore end hand grip). This also enables you to slip a pellet into the breech at the same time, making break barrel guns quick and easy to load and use, as long as you are strong enough to compress the spring. When you slip the trigger, the spring propels the piston down the air chamber, compressing the air to very high pressures before releasing it into the breech behind the pellet. As the spring expands, a lubricant on it (usually molybdenum paste) sprays onto the wall of the air chamber. When this is compressed, it diesels, adding a little more power. A gas ram gun works in exactly the same way, but you compress an inert gas inside a sealed cylinder rather than a spring. A gas ram is smoother than a spring, more consistent and less "twangy". It does recoil, but in a totally different manner to a spring gun. (No twangy extension and no relatively uncontrolled dieseling). Also, the gas ram does not lose tension with age like a spring does. NB You can buy a DIY drop in gas ram (below) for many spring guns- contact your manufacturer, or Theoben, who do gas struts for most popular guns. See "Links" page, or e-mail me for further details. (Be warned, however, that there have been some queries over the quality of the drop in gas rams).
Go on! You know you want to!
The Theoben gas strut