There is no reason that an air rifle could not be produced with a bore of any reasonable size, but because it requires a greater volume of compressed air to launch a projectile of large diameter, a large reservoir would be required. The result could be an air cannon that fires pumpkins. For many years the most common airgun calibers have been the .177, .20, .22, and .25 calibers. In addition to these calibers, a few very powerful precharged pneumatics fire projectiles of 0.355-0.357 (9 mm) diameter, but they are much less common than those of the smaller calibers. When it comes to selecting an air rifle for hunting small game, several factors should be considered.
First, pellets are much more readily available in .177 and .22 calibers. The .20 caliber has been identified with the excellent Sheridan multi-pump rifle since its introduction in 1948. As the power plants in air rifles became more powerful, the .25 has increased in popularity, but most sporting goods stores do not routinely carry pellets in this caliber. Also, the selection of pellet types is not as extensive as in the smaller calibers, and .25 caliber pellets are considerably more expensive. However, since the number of shots fired in hunting situations is usually limited, the cost of the ammunition is not a major concern. Once a supply of pellets is acquired, the hunter is not required to make a trip to a sporting goods store before going afield.
The major issues with the selection of an air rifle for hunting are those of power and accuracy. Most air rifles used in competition are low-powered .177 caliber models, and some shooters believe that rifles in that caliber are generally more accurate. However, rifles such as the Nitro Piston® and Benjamin Discovery and Marauder deliver excellent accuracy in both .22 and .25 calibers. In fact, when equipped with a good scope these rifles are sufficiently accurate to take small game as far as one should try to shoot game with an airgun. Moreover, the hunter who has good skill and patience can hunt using a Benjamin or Sheridan multi-pump, as has been done for generations. With their barrels attached full-length to a pump tube of large diameter, these multi-pump rifles are capable of excellent accuracy.
Generally, multi-pump rifles have an upper limit to power of about 12 ft lbs at the muzzle. This is increased to about 16 ft lbs for the break action models. The power king is the PCP which may produce as much as 25-30 ft lbs making such a rifle the clear choice for hunting larger species.
From an analytical point of view, animal tissue must be displaced to cause trauma that will result in instant immobilization of the animal. The result is that the larger the hole, the greater the amount of tissue damage. Although a .177 caliber pellet in a lethal spot such as brain or spine will dispatch a small animal quickly, the greater impact of a pellet of larger diameter and weighing perhaps three times as much is even more effective. The major factor is always placing the pellet in exactly the right spot. However, the issue is not just dispatching the species but doing so quickly and humanely. It is better to have power in reserve than to use an air rifle that is not suitable for the game. So what is the best type of airgun for hunting? A good .22 or .25 caliber will certainly get the job done on small species, but a potent .35 caliber PCP would do even better and would permit taking larger species.
Jim House began shooting with a single shot BB gun at a very early age. Now, seventy years later, he is an airgun enthusiast. After a 32-year career as a chemistry professor at Illinois State University, he has written extensively about shooting sports, which has resulted in the books American Air Rifles and CO2 Pistols and Rifles. His books also include The Gun Digest Book of 22 Rimfire and, with his wife Kathleen, Customize the Ruger 10/22. Jim is the Reloading Editor for Gun World magazine and a Contributing Editor for The Varmint Hunter Magazine, The Backwoodsman, Airgun Hobbyist, and The Illinois Shooter. Although a lot of his work is with firearms, he maintains a keen interest in airgunning while also serving as Adjunct Professor of Chemistry at Illinois Wesleyan University.