I found a way to do this, i believe somewhere on this forum..here's the code
_faceheight = 1.80;
_dir = getdir unittohit;
_dir = _dir - 180;
_px = getpos unittohit select 0;
_py = getpos unittohit select 1;
_npx = _px + (2*(cos _dir));
_npy = _py + (2*(sin _dir));
_dir = _dir + 180;
_bullet = "B_556x45_Ball" createvehicle [_npx,_npy,_faceheight];
_xvel = (1000*(cos _dir));
_yvel = (1000*(sin _dir));
_bullet setVelocity [_xvel,_yvel,0];
Play with the direction, this way the bullet comes from behind and you can actually see it hit the enemy
Adjust the _faceheight, _npx, _npy to hit other bodyparts...
Chanching the 1000 will affect the speed of the bullet...this is an instant kill when hitting the body/head...with a value of around 200 this is not always the case, so also change that if the effect isn't good;)