Chairgun was a very weak ballistics program anyways. The only advantage I can think of that Chairgun had was a library of BC's for pellets. Other than that, the program seems like a dinosaur when compared to other ballistic programs.
If you know the BC of the pellet you are using (generally G1 or G7), you can use any of the great ballistic apps that were created for powder burners that are out there. Applied Ballistics, Trasol, Hornady has a 4 DOF ballistics calculator, Lapua has a 6 DOF calculator, etc. Plenty of solid options.