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 shoots pumpkins.
For many years, the most common air rifle calibers have been the .177, .20, .22, and .25 calibers. In addition to these calibers, a few very powerful pre-charged pneumatics shoot projectiles of .35 to .50 diameter, but they are much less common than those of the smaller calibers. When it comes to selecting the right pellet gun caliber for hunting small game, several factors should be considered.
Pellets & Ammo
First, pellets are much more readily available in .177 and .22 calibers. The .20 caliber has been identified with the excellent Sheridan multi-pump air rifle since its introduction in 1948. As the power plants in air rifles became stronger, the .25 has increased in popularity, but most sporting goods stores do not routinely carry pellets in this air rifle 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 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.
Power & Accuracy
The major issues with selecting an air rifle for hunting are those of power and air gun accuracy. Most air rifles used in competition are low-powered .177 caliber models, and some shooters believe that air rifles of that caliber are generally more accurate. However, air rifles, such as the Nitro Piston®, Benjamin Discovery, and Marauder, deliver excellent accuracy in both .22 and .25 pellet gun calibers.
In fact, when equipped with a good scope, these air rifles are sufficiently accurate to take small game as far as one should. 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, the air gun calibers of these two are capable of excellent accuracy.
Generally, these multi-pump air 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 an air rifle the clear choice for hunting larger species.
Impact
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 air rifle caliber shot in a spot such as the 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 air rifle caliber for hunting? A good .22 or .25 caliber pellet gun 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 down.