I'll add more detailed review of my air rifles and scopes in the future but below is a quick summary of my personal opinions and views based on my experience and ownership of a variety of rifles. Air rifle choice, and brand loyalty, seems a very emotive topic with opposing views causing many arguments on the internet forums and I know anything I say below will be agreed with by some and just as strongly disagreed with by many others. 

All my views below are based on United Kingdom (more specifically England and Wales) where, as at 2016,  the maximum power of an unlicenced air rifle is 12 foot-lbs.  More powerful air rifles can be purchased and used but these require the user to hold a Firearms Certificate (FAC) and is outside the scope of this article.

 

Calibre Choice

The power of an air weapon is measured in muzzle energy (or kinetic energy). This is the energy at which the projectile (Pellet) leaves the muzzle of the air weapon.
 

Air rifle power is calculated as a function of the weight of the pellet and the speed of the pellet. A smaller, lighter,  pellet can therefore be fired at a greater speed and stay within the UK, limit than a larger, heavier pellet.  As soon as a pellet leaves the muzzle of the rifle it starts to fall vertically under the influence of gravity. The longer the pellet is in the air the further it has fallen before it hits the target.  It follows therefore that a faster pellet arrives at the target sooner and has fallen a shorter vertical distance. Given that all the pellets have to comply with the UK power limits

The  examples below show the typical weight (in grains) and speed (in feet per second) of a pellet in various calibres when all are fired at 11.5 ft lb power (Probably as close to the 12 ft lb limit as you want to go to give a margin for error).  Remember that, in the UK, if your rifle exceeds 12 ft lb in power, and you don't have a Fire Arms Certificate, you have committed a serious Fire Arms Offence which can result in a custodial sentence. The table also shows the time for the pellet to reach the target at a distance of 25 yards and how far the pellet has dropped below the muzzle of the rifle by the time it hits the target at a variety of distances.

Calibre

Pellet

Weight

Grains

Speed

FPS

Time

To 25 yard

Target

Seconds

Pellet
Drop

in inches

at 10 yards

Pellet
Drop

in inches

at 25 yards

Pellet
Drop

in inches

at 45 yards

Retained

Energy

at

45 yards

ft lb

.177 8.44  783.5  .103 0.3  2.00 6.9 6.84
.20  13.73  614.3  .128 0.5  3.10 10.5 8.12
.22  15.89  571  .138 0.5  3.50 12.1 8.21
.25  25.39  451.7  .174 0.9  5.7 19.3 8.28

 Examples Above all fired at a power of 11.5 ft lb

Examples below using a lighter weight .22 pellet or more power      

.22 13.43 621.1 .127 0.50 3.0 10.30 8.03

.22

@21.65ftlb

15.89 783.5 .101 0.30 1.9 6.5 15.15
               

 

pellet trajectory comparison

 

 

Calibre choice - .177 or .22

I'll ignore the .20 and .25 calibres as if you are considering these then you probably already know enough to make your own decisions and have your own views.

Accuracy

All calibres can be as accurate as one another.  It is your ability to estimate target range accurately and make the appropriate adjustment to your zero point that makes the difference.  In general a lighter .177 pellet will have a flatter trajectory, when fired at UK air rifle power levels, so will be more forgiving of range estimation.  A heavier, slower, .22 pellet will have a more curved trajectory so will require more adjustment of hold over/ hold under for different ranges than a .177 pellet fired at the same power level but it is no less accurate when zeroed in, it just requires more skill from the shooter.

In the UK, unless you go to FAC power levels (again if considering this you will already have your own expert opinions) then .25 is really only suitable as a short range vermin control calibre (at which it excels) as the slower speed of the pellet (due to the UK 12ftlb power limit for non FAC air rifles) gives a very curved trajectory. 

I did consider going for the .20 calibre as a happy medium between .177 and .20 but there is currently a more limited choice of pellet in the .20 calibre and I can either use a light weight .22 pellet or a heavy weight .177 pellet to obtain nearly the same trajectory as a .20 calibre.  .20 does have it's devotees however and for some this may be the ideal do everything calibre.

If you are considering entering competitions, unless you specifically want to enter a .22 class competition, then .177 is the calibre of choice. The higher speed of the lighter .177 pellets give a flatter trajectory