Half guns and artilleries

In the world of air rifles, there are giants, there are dwarfs and then there are halflings.

Giants are rifles that are longer than usual. For example, all the Hatsan "long" rifles are longer than usual. Dwarfs refer to rifles that are shorter than most, this include longer bullpups that are 30 inches in overall length. Halflings refer to very short rifles like the Edgun Lelya, Taipan Mutant/Veteran Short, and interestingly the Edgun Leshiy.

Accuracy

A longer rifle has more mass. More importantly, it has more moment of inertia because there is more mass that is further away from the center of gravity. Moment of inertia refers to the resistence to angular acceleration. In plain terms, moment of inertia is what dampens movement while aiming.

This means that a longer rifle is easier to aim accurately. If open sights are used, then the arguments applies doubly because the sight radius is longer, allowing improved precision in addition to stability.

Let's use some real examples. A Hatsan BT65 long barrel rifle has 200% the overall length and 200% the weight of a Edgun Leshiy. The strange part is that moment of inertia (of a mass point) is proportional to the square of distance from rotation pivot. Loosely translated (my calculus isn't as good as it used to be!), the BT65 has about 400% the stability of an Edgun Leshiy.

Moment of inertia helps not only aiming, but also the recoil effect. Every air rifle recoils due to the conservation of momentum. Momentum is proportional to mass times velocity. The BT65 long barrel experiences only 25% of the recoil effect compared to an Edgun Leshiy assuming the projectile mass and muzzle velocity remains the same.

All things being the same, a longer rifle makes for a more accurate rifle.

However, the price of a longer rifle is weight and overall length. Weight is an important factor when a rifle needs to be carried. Overall length can be an issue for carrying as well. A heavier rifle is also more straining for unsupported shooting positions.

Efficiency

Efficiency is defined as the proportion of projectile energy versus energy released. A longer barrel is, generally, more efficient because there is more length for a projectile to accelerate. However, this argument only applies up to a certain point. When the air pressure behind a pellet is reduced, due to expansion, to the point where the force acting on the pellet equals the frictional force rubbing against barrel wall, then a pellet is no longer accelerating. Any length beyond that is only going to decelerate a pellet and decrease efficiency.

Piston rifles (spring or gas piston) do not benefit from longer barrels as much as pneumatic rifles.

Ease of carry

The biggest advantage of a shorter rifle is easy of being carried. A Hatsan BT65 long barrel has an overall length close to 50". By comparison, an Edgun Leshiy is a break barrel design that only has a overall length of 14". In between, an Edgun Lelya has an overall length of about 20", a Taipan Mutant/Veteran Short is about 23" long. Various "full-size" bullpups range from 26" to more than 30". Short carbines start at about 35".

Some rifles are classifed as TDRs (take down rifles). Such rifles disassembles to parts that fit in a small case. While such rifles are easy to carry, they may take a significant amount of time to reassemble.

Energy

In general, longer rifles can deliver more energy because there is more room for a projectile to accelerate. Even short PCPs can deliver 20 FPE or more. However, if 50 FPE or more is needed, long rifles are the only options.

Shot count

There is no contest in this department. Everything else being the same, a longer rifle has more air tube volume, more barrel for acceleration, and therefore more shot count.