When deciding on which calibre to use there are many factors to consider. A .177 pellet travels at a greater velocity than the heavier .22 when fired from guns of identical power, but this does not mean that .177 is more powerful. For example;
.177 pellet weighing 8.4 grains, fired at 802 fps (244mps) = 12 foot pounds of kinetic energy.
.22 pellet weighing 16 grains, fired at 581 fps (177mps) = 12 foot pounds of kinetic energy.
The lighter .177 pellet has a flatter trajectory which means that when aiming less hold over is required than with a .22 over normal airgun ranges. The benefit of this is that targets are easier to hit with a .177 even if there are slight inaccuracies in range estimation; this is why .177 is the calibre of choice in Field Target and paper target disciplines.
In countries with laws limiting airgun power, .177 is gaining popularity for hunting, as their flatter trajectory is seen as an advantage over .22. Where .22 scores over .177, is in their retention of down-range energy.
At the UK power level of 12ft/lbs the difference is fairly small, but as the power increases, the trajectory of both calibres flattens out so reducing the advantage that .177 would have over .22 at lower velocity. However, when a pellet approaches the speed of sound (which is about 1100 feet per second), it can become unstable, resulting in poor accuracy. This problem is almost exclusively confined to .177. The .22 really comes into its own as the velocity increases, when it’s heavier weight means that the pellets retain greatly superior down range-energy, and do not suffer from the loss of accuracy that can affect high velocity .177 pellets.
Do not confuse velocity with power, they are not the same. Velocity is simply a measure of speed; power as measured in foot pounds or joules, is a measure of kinetic or striking energy, which takes into account the weight of the pellet as well as its speed. This is illustrated in the graph below which shows the required velocity of both .177 and .22 pellets for them to have the same striking energy or power, measured in foot pounds.