Altaïr
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How do you calculate drag?
ρ = 1.225 kg/m^3 : air density. That's at 15°C at sea level (air density varies with temperature and altitude), though you can use this value, it will give you a good idea.
S = π×R^2, where R is the radius of your projectile (it's a sphere right?): cross sectional area
v: the projectile speed: 25 m/s for you.
Cd: drag coefficient. For a sphere in those conditions, you can assume it's 0.5.