Ultrasonic cleaner

You can find several of these units on Ebay for your air rifle parts cleaning.
Their prices usually it does not exceed 100$ for a 2lit-3lit cleaner.
Also you need a cleaning solution depends the material.
Usually they used for
* scientific & chemical labs for cleaning test tubes
* medical and dental clinics for cleaning dental drill, dental forceps during sanitizing
* jewelry manufacturer & workshops and even gem stores for cleaning in-process and finishing necklace, finger rings, earrings, etc.
*electronics workshops, watchmakers, cellular phone repair workshops and service center (where it could be used for cleaning a phone that has been exposed to enough moisture to hinder its operation)
*precision machinery repair in cleaning pneumatic valve, hydraulic directional control valve, overflow valve, fuel injection nozzle, precision gear driving sets and bearings, the excellent cleaning result can achieve, the deepest dirt and the grease inside the tiny lacuna can be cleaned out and after refresh the new lubricating grease, like giving the new life to these parts, in cleaning optical items and many more.

Also you can wash your pellets for accuracy

Here it is a video to see how it works: