Are they moles or are they voles or something else (i.e., ground squirrels gophers, Norway rats, etc.)?
Specific to moles, there are about 39 different species world wide with seven different species in the US. Some are protected while most aren't but you need to find out what you've got before trying to control them if you want to be successful. The issue with using air rifles is that most mole species either never or almost never go above ground and when they do, it is literally for seconds. So unless you are in the right place with your finger on the trigger at the right time, don't put too much success into mole control with air rifles.
For actual control, the best option is almost always specific mole traps based on what species and soil conditions you have. The propane control is done with a device called the Molenator and is illegal to use in most residential areas due to safety risks to people and homes as are most fumigants in the US. Most poisons don't work either as moles belong to the family Talpidae and are insect eaters while voles and such belong to the order Rodentia and are plant eaters. So putting poisoned peanuts down the hole isn't going to be effective as the moles are looking for earthworms and grubs, not legumes.
Anyhow suggestions would be highly appreciated! 
