What is the formula for the distance a projectile will travel with a constant initial velocity?

I am going to make an automatic canon in the sandbox game gmod, but I have never taken a physics cl. The projectile will have a constant initial velocity, and will usually be fired on a map with level ground (though I may modify it later for uneven ground as well). The only thing I can controle is the direction and the angle at which it is fired. Thanks for the help :D.