Oh puhleeze... mometum is in the x and y coordinate?? Say what? A projectile travels in an arc, called a parabola. The path of a parabola can be depicted on an coordiante plane, using x and y coordinates. I know that sounds condesending, only because it is.

Velocity is a vector quantity. Since momentum is the product of a scalar quantity (mass) and a vector quantity (velocity) it to is a vector quanity. The same argument can be made about kinetic energy.

Since a projectile travels in a parabolic path it does, in fact, have horizontal and vertical components. The horizontal compent of velocity on a projectile is constant. The vertical component of velocity, however is not (a constant, the gravity thing). True, both the vertical and horizontal components of velocity can be calculated as a fucntion of the intial velocity of the projectile and the angle at which it was fired.
As such, both the vertical and horizontal components of momentum can be calculated as well. The question becomes, so what??
When two projectiles strike a target, the projectile with the greater momentum, p=mv,(all else being equal, diameter, coefficient of friction, etc., etc.) will have the greatest penetration, end of report.