I've tried a few Android apps, but I haven't really spent enough time on them to say how good or bad they are.

Airgun Helper Mobile 3 Seems to do a bit of everything, the pellet database is very handy, as is the gunrack, though very US oriented. If I could concentrate on it for a day while at the range, with a chrono and proper measuring tools, I might do it justice.

Airgun Wizard Pyramid Air primarily a calculator app, but useful.