Re: shooter app
I highly suggest you do some reading and searching because the answer is not that simple. Ballistic calculators are shit in, shit out programs; your output data is only as accurate as your input data.
Nearly every variable in shooting is constant and can be accounted for(except wind), but the information has to be accurate. I suggest you do some reading on ballistics and find out exactly WHY and HOW the different factors affect your bullet. With a better understanding of these factors, it will make the calculator less intimidating.
Key factors to pay attention to:
Muzzle velocity
barrel length
barrel twist
bullet weight
ballistic coefficient(the G7 scale is a more accurate drag model for modern projectiles than G1 and beware of mfrs posted BCs as they're a selling point and usually embellished. If possible, use the projectile in the library marked with "Litz". If you don't know who he is, google Brian Litz)
scope height over bore
As far as environmentals,
Altitude
barometric pressure
OR
density altitude
temperature
wind speed(input is at the shooter but must be tracked at several points downrange using indicators)
wind direction(same deal as above)
These are the absolute bare minimum to calculate an accurate firing solution. Again shit in, shit out so if you don't know it, don't swag it.