Generally speaking a longer barrel is going to allow for more power. If you compare most CO2 pellet pistols to CO2 Pellet Rifles, you will find that the CO2 Pellet Pistol will generally shoot about 300-400 fps using a standard lead Pellet. On the other hand many CO2 Pellet Rifles can get up to as high as 600 fps which is much better speed for humanly shooting some smaller pests.
If you are talking about Springer or Variable Pump Air Pistols versus their Air Rifle counterparts, again the Air rifles will generally shoot with much more power. I have seen some none CO2 Air Pistols shoot up into the 500-600 fps range but again compered to none CO2 Air Rifles, they can even double these numbers.
Some of you may ask about Steel BB Pistols and Rifles. I personally would never really sue this type of ammunition for pest control simply because most BB Rifles have much less raw power than Pellet Rifles and steel BB’s will not be as accurate at any type of longer distance.
When holding any rifle from the shoulder, you have three point of contact with your body spread out in a much longer distance than a two handed pistol grip. Holding steady even without a rested position is much easier using a rifle. Even when using a rested position, the rifle will more stable.
A big factor in accuracy is stability but there are also other factors to consider. Barrel length its self does not make the gun more accurate, you only need a few inches of barrel length to get constant accurate results from a gun. What makes longer barreled guns more accurate is the distance between the rear and front sight. The closer the sights are together, the more margin for error there is, the longer you spread out the sights from each other, the more accurate the sighting system becomes.
Overall More Humain:
When you consider Air Rifles tend to have higher power, better accuracy and overall more consistency than shorter barreled Air Pistols. The logical choice for humanly shooting a Pest is to use the Air Rifle. The last thing you want to do is make any animal suffer, even if it has been causing a lot of havoc around your home or farm or place of business.