Where:

 V = initial muzzle velocity (pixels/iteration)
 initX,initY = initial position of projectile (ie. gun's position)
 t = current time
 a = acceleration due to gravity (0.5 pixels/iteration)

 Including air resistance:
 "Decelerating component proportional to v-squared and a component proportional to v. The former is much larger."
 "To take into account wind drag, add to the horizontal distance moved a small extra distance term proportional to the wind speed at each time step."

 newX = initX+((V*Math.cos(h))*t);
 newY = initY+t*(V*Math.sin(h)+a*t/2);
 
Configuration:
 
 v = 20;                    // velocity
 var initX = mouse.clientX; // start X point
 var initY = mouse.clientY; // start Y point
 var t = 1;                 // time
 var a = 0.5;            // gravity
 var h = 5;               // angle