This does seem to work, I think it will give a good idea of what your rifle is shooting at. Stolen from another AR site.
Chrono Connect Lite
The Lite version has all the basic functionality you need to read the speed of a shot from your gun. You need to enter 3 distances, the pellet weight and the pellet BC value ( Most available at ChronoConnect.com ) and you can begin shooting.
The app will display the speed of the pellets, the power it equates to and the min and max values recorded for each.
The Pro version has many more cool features such as,
- Unlimited shots in one go ( no 3 shot 'Lite' limit ).
- Full list of shots in the string.
- Graph of shots in the string.
- Full shot and string management.
- Comprehensive pellet database ( no need to manually enter weights and BC values for most pellets )
- Comprehensive rifle and pistol database.
- Visual over power warnings.
Lite is free and this video shows how to setup and use!