Pellet guns shoot a projectile of very low mass, hence it is both quick and easy to hand pump sufficient air pressure for each shot to propel the small pellet at relatively high velocity from an air rifle. A pneumatic speargun, or a variation on this design such as the hydropneumatic speargun, have to propel a spear shaft of considerable mass, so much more air pressure is required. The pressurised air in the pneumatic speargun acts as a strong spring which is never fully released except for occasional maintenance like changing the lubricating oil inside the gun. The expellable air or carbon dioxide gas gun requires access to a recharging facility which can supply high pressure gas, once you run out of gas you have no more shots. Muscle power loaded spearguns are only permitted in spearfishing competitions and this has trickled down to determine the availability of spearguns for recreational use. Some countries ban expellable gas weapons of any type by declaring them to be prohibited items.
If you look through the archive http://forums.deeperblue.com/archive/index.php/f-110.html then you will see advanced pneumatic spearguns discussed such as the "Mirage", the "Mamba" and the "Black Sea" hydropneumatic. These guns have all advanced the shooting performance of spearguns, but for most spearfishing tasks you cannot beat the "Sten" type gun from which most modern pneumatic spearguns have been derived.
Why are more short pneumatic spearguns available in stores than longer models? Mainly because they are easy to load by just about anybody, while longer pneumatic guns are more difficult because they require a longer arm reach and more strength from the operator. You could say exactly the same thing for very long band guns. The propulsive stroke in the gun and the average force level applied to the shaft over that distance determines the shooting performance of any speargun, not the physical length of the gun itself, although that does affect the mass of the projectile, i.e. you have a longer spear shaft in a longer speargun.
Longer pneumatic models need to be ordered through your local dealer, dive shops usually stock fast moving items rather than ones that tend to sit on the shelves.
If you look through the archive http://forums.deeperblue.com/archive/index.php/f-110.html then you will see advanced pneumatic spearguns discussed such as the "Mirage", the "Mamba" and the "Black Sea" hydropneumatic. These guns have all advanced the shooting performance of spearguns, but for most spearfishing tasks you cannot beat the "Sten" type gun from which most modern pneumatic spearguns have been derived.
Why are more short pneumatic spearguns available in stores than longer models? Mainly because they are easy to load by just about anybody, while longer pneumatic guns are more difficult because they require a longer arm reach and more strength from the operator. You could say exactly the same thing for very long band guns. The propulsive stroke in the gun and the average force level applied to the shaft over that distance determines the shooting performance of any speargun, not the physical length of the gun itself, although that does affect the mass of the projectile, i.e. you have a longer spear shaft in a longer speargun.
Longer pneumatic models need to be ordered through your local dealer, dive shops usually stock fast moving items rather than ones that tend to sit on the shelves.