Hypothetical and untested...
(edited)
Code:

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define _tm1, skill29;
	// world speed
define _wSpeed, skill30;
define _wSpeedX, skill30;
define _wSpeedY, skill31;
define _wSpeedZ, skill32;
	// relative speed
define _rSpeed, skill33;
define _rSpeedX, skill33;
define _rSpeedY, skill34;
define _rSpeedZ, skill35;
define _prSpeedX, skill36;	// parent speed (relative)
define _pwSpeedZ, skill37;	// parent speed (world)

function create_balls() {
	proc_kill(4); 	
	vec_for_vertex (temp, weapon1, 1465);
	ent_create (ball_mdl, temp, move_balls);
}

function move_balls() {
	my.pan = camera.pan;
	my.tilt = camera.tilt; 
	if (player != NULL) {
		my._prSpeedX = player._rSpeedX;
		my._pwSpeedZ = player._wSpeedZ;
	}
	while (me != null)  {
		my._tm1 += 0.0625 * time_step;
		my._rSpeedX = max(0, (5 + my._prSpeedX) - my._tm1 * 0.25);  
			//tD: note: the ball may slow down too fast
		my._wSpeedZ = (-5 + my._pwSpeedZ) * my._tm1;						
			//tD: the ball may start to fall too fast
		if (c_move (my, my._rSpeed, my._wSpeed, ignore_passable) <= 0.01) {
			break;
		}
		wait (1);
	}
}

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