Buy a real pellet gun. They're:
- Less than 1/3rd the cost of airsoft
- 10x more accurate
- Carry up to 9 times more energy to punch in to your target
For your requirements, airsoft is your only real viable choice if you must absolutely have a full auto gun, however, given the issues I've indicated above, an airsoft gun does not carry nearly enough energy to seriously wound or maim a pest (except for maybe a cockroach.) Firing almost any airsoft gun at an animal would do nothing more than to cause it unnecessary pain and suffering.