One of the more common air-gun questions we get at Yahoo Answers is which caliber is better, .177 or .22. Whenever that question is asked, there's always a lot of misinformation and disinformation that gets spouted in addition to the usual heckling you see on any air-gun question. There are also a couple people who make an honest attempt to explain the complexities of the issue and help people to make an informed choice. This essay is intended to help clarify the topic in a format which does not have a character limit and hopefully reduce the controversy about which of the two most popular airgun calibers is "better" by examining the advantages and disadvantages of both calibers.
Unlike firearms, an air-gun's ammunition does not supply any of the projectile's power. The air-gun's power plant is the only source of power for the projectile. So if you take two airguns of the same make and model that differ only in caliber, they will usually produce almost identical power levels with 1 ft-lb or less of muzzle energy separating the two calibers. To prove that the power difference between calibers is minimal, lets look at typical chronograph numbers for a hunting air-gun, the RWS 34.
There is an exception to this generalization with pre-charged air-guns, particularly the more powerful pre-charged airguns, since they are at their most efficient when shooting heavier pellets which can give an energy advantage to .22 caliber guns. However this rule of thumb holds true for spring-piston air-guns, co2 air-guns, and pump pneumatics. So the notion that .22 caliber guns are inherently more powerful is generally false.
While I have no doubt that the this is true when it comes to drugs, in the world of airguns, this is another myth. Speed does not kill. Impact energy, destruction of major organs, and bleeding from the wound channel is what kills game. Velocity is one part of kinetic energy, so it does contribute to the amount of impact energy. However it is only part of the equation. As a result its possible to have a light, high-velocity pellet and a heavy, low-velocity pellet with the same, or very nearly the same, impact energy. Again lets look at some numbers.
So by looking at the energy figures in that table, its apparent that velocity by itself does not make an airgun any more deadly.
An additional concern with high velocities when shooting airguns is the negative impact that velocities over 1000 fps have on accuracy. In these "trans-sonic regions", the high drag nature of airgun pellets tends to produce poor accuracy because of how aerodynamics change as you approach the speed of sound. Its sort of like how Chuck Yeager complained that the X1 was shaking a whole lot as it approached the speed of sound. This "shaking" results in larger groups and poorer accuracy. So its actually advantageous to keep pellet velocities in the 800-1000 fps range, well below the speed of sound.
.177 for Feathers & .22 for Fur:
This is a common catch-all you'll hear when someone asks the .177 vs. .22 question. And there is some truth to it. However it radically over-simplifies things. The idea behind this saying is that that lighter .177 caliber pellets are more effective for small, light bodied animals like birds while the heavier .22 caliber pellets are more effective with mammals like squirrel and rabbit because it leaves a bit larger wound channel. And it might even have been true back in the 1950s and 1960s when air-gun power levels hovered around 5-8 ft-lbs of muzzle energy. However today things have changed.
Today it really doesn't make a lot of difference whether you're using .177 or .22 caliber for most small game (squirrels & rabbits), nuisance birds (English sparrows, starlings, and feral pigeons), or vermin (mice & rats). That's because the energy levels for the "adult" air-guns people use for hunting run from a low of around 5 ft-lbs of muzzle energy up to 80 ft-lbs of muzzle energy. The average spring piston gun (the most common choice for hunting due to its affordable price) seems to be about 12-17 ft-lbs of muzzle energy. In addition, the pellets we have today are usually better quality and perform better than the ones from the 1950s and 1960s. So not only do today's guns deliver more energy, but they also deliver more accuracy, which lets people place pellets to make better use of the energy their guns deliver. And to be perfectly honest, x ft-lbs of impact energy is x ft-lbs of impact energy regardless of whether its delivered by a .177 or .22 pellet.
The only time when caliber does become an issue is when you're using an airgun for larger than normal game, like raccoons, opossum, nutria, or fox. With animals that are that big and tough, .22 caliber pellets are a better choice. That's not necessarily because of any inherent advantage, simply because very heavy .22 (or even .25) caliber pellets shot at high velocities from the most powerful "magnum" spring-piston and more powerful pre-charged air-guns, which are the only appropriate choice for animals that large, can and do let you deliver a lot more impact energy without pushing the pellet to transsonic and supersonic velocities.
Having already stated that both .177 and .22 caliber pellet guns can be used to kill small game animals, its important to note that both calibers have strengths and weaknesses. And so choosing a caliber is necessarily a matter of compromises. Which caliber to pick is in part a matter of which compromises you're willing to make and in part a matter of matching the pellet to the power of the gun you're looking at.
.177 caliber guns shoot a lighter, smaller diameter pellet at a higher velocity. This produces a flatter trajectory at realistic airgun ranges, say out to 40 yards. This flatter trajectory reduces the importance of range estimation and makes it easier to precisely place a pellet since hold-over/hold-under becomes less important. In addition, .177 caliber offers two other advantages. The first is that since it is the most common airgun caliber, it has an extremely wide selection of pellets. This increases the odds of finding a pellet your gun shoots accurately. The second is that because .177 pellets use less material and are made in such huge numbers the cost is usually much less than a .22 caliber pellet. This difference in cost is several cents per each pellet. So if you're planning on shooting a lot, either in formal matches or plinking, .177 caliber is very appealing. There are two disadvantages of .177 caliber: a narrower wound channel and tendency to over-penetrate on light-bodied animals like nuisance birds. Both are related to the fact that the .177 is a smaller diameter projectile. The narrower wound channel means that with body shots .177 caliber will do less internal damage and correspondingly take longer to allow an animal to bleed out. The over penetration issue stems from the fact that the impact energy is spread over a smaller area. Neither disadvantage is insurmountable, especially if you limit yourself to head-shots and use pellets which are optimized for the kinds of game animals you're shooting.
.22 caliber guns shoot a heavier, larger diameter pellet at a lower velocity. As a result of their greater diameter, they will produce a larger wound channel and are less likely to over-penetrate. The larger diameter wound channel means they're going to kill more quickly with a body shot. And the fact they are less likely to over-penetrate means they actually may be a better choice for nuisance birds. There are two big disadvantages to .22 caliber pellets though: trajectory and cost. In guns producing less than 20 ft-lbs of muzzle energy, .22 caliber pellets result in a more pronounced trajectory which means range estimation has to be more precise than with a .177 caliber gun producing less than 20 ft-lbs. In guns producing over 20 ft-lbs, trajectory issues largely go away. The other disadvantage is cost... The cheapest .177 pellets run about $.01 per shot. The cheapest .22 pellets cost about $.03 per shot. So if you plan to shoot a lot, .22 will cost significantly more over the life of the gun.
The 20 ft-lb Rule:
Its important to reiterate that both .177 and .22 caliber guns can be used to take small game (squirrels & rabbits), nuisance birds (English sparrows, starlings, and feral pigeons), or vermin (mice & rats). That said, I think the advantage between the calibers shifts at around 20 ft-lbs of muzzle energy because of issues with trajectory and pellet velocity.
In guns producing under 20 ft-lbs of muzzle energy, .177 caliber has the advantage. The reason is that the flatter trajectory aids more accurate shot placement in the field. This stems from the fact that unless you're using some form of range-finder, its all too easy to be off a bit when you estimate the distance to a rabbit or squirrel. The more pronounced trajectory of a .22 requires a bit more compensation which is more likely to result in either a bad hit or a miss if your range estimate is off. So personally I'd rather minimize the chances of that by going with a .177 caliber in guns producing less than 20 ft-lbs.
In guns producing over 20 ft-lbs of muzzle energy, I think the advantage shifts to .22 caliber. At that point the trajectory is sufficiently flat that .177 caliber doesn't offer any substantial advantage. And just as importantly, in guns producing more than 20 ft-lbs of muzzle energy, the heavier .22 pellets let you keep the velocity sub-sonic where accuracy is best unlike .177 caliber pellets which will hit the transsonic region due to their lighter weight. As a result, accuracy tends to be better, especially when you look at guns producing 25, 30, 40, or more ft-lbs of muzzle energy.
Which Is Better?
If you're like me, at this point you probably want a simple answer to the question which is better. As I've stated throughout this essay, both calibers can reliably take small animals up to the size of rabbits at reasonable airgun ranges. And in the hands of an experienced shooter, both calibers can be surprisingly accurate. So while certain calibers may have an advantage at a given power level, which is best ultimately becomes a matter of personal preference. With that truism in mind, I encourage you to consider the information presented here as an introductory primer to help you make a good choice.