What is the best coordinate system for describing the projectile motion? Rectangular coordinate system or n-t(normal and tangential) coordinate system.
If the range is not too large, you can approximate the acceleration (due to gravity) vector to have the same direction over the trajectory of the projectile. It will probably be a good idea to use earth fixed 2D cartesian coordinates in this case, one axis horizontal and the other vertical to the Earth's surface. The acceleration acts only along the vertical direction. The motion projected onto the horizontal direction becomes free of acceleration.