I have a simple code that shoots a rocket out of a weapon with the expected position being the center of the screen. I use vec_to_angle to alter the pan and tilt of the rocket when it leaves the weapon barrel (vec_for_vertex) and move it forward along the x direction using a c_move instruction:

Code:
VECTOR temp;
vec_set(temp, vec_for_screen(vector(screen_size.x/2, screen_size.y/2, 10000), camera));
vec_to_angle(rocket.pan, vec_diff(NULL,temp.x,rocket.x));

while(1){
  c_move(rocket, vector(4*time_step, 0, 0), nullvector, IGNORE_FLAG2);
  wait(1);
}

However, with the above technique, the rockets do not hit the direct center of the screen but arrive slightly off-center:

It appears the offset is affected by the model alignment despite using vec_to_angle:

Code:
#include <acknex.h>
#include <default.c>

BMAP* center_bmp = "center.bmp";
PANEL* center_pan = {
    bmap = center_bmp;
    flags = SHOW;
}

void shoot_rocket(ENTITY* rocket) {
	set(rocket, FLAG2);
	vec_for_vertex(rocket.x, player, 987);
	
	VECTOR temp;
	vec_set(temp, vec_for_screen(vector(screen_size.x/2, screen_size.y/2, 10000), camera));
	vec_to_angle(rocket.pan, vec_diff(NULL,temp.x,rocket.x));
	
	while(1){
		c_move(rocket, vector(4*time_step, 0, 0), nullvector, IGNORE_FLAG2);
		wait(1);
	}
}

void player_action() {
	player = me;
	while(1) {
		camera.pan -= mouse_force.x;
		camera.tilt += mouse_force.y;
		
		vec_set(camera.x, player.x);
		camera.pan = player.pan;
		camera.tilt = player.tilt;
		
		if(mouse_left) {
			ENTITY* rocket = ent_create("rocket.mdl", nullvector, NULL);
			shoot_rocket(rocket);
		}
		
		wait(1);
	}
	
}

void main(){
	level_load("level1.wmb");
	wait(1);
	
	center_pan.pos_x = (screen_size.x-bmap_width(center_bmp))/2;
	center_pan.pos_y = (screen_size.y-bmap_height(center_bmp))/2;
	
	ent_createlocal("rocketgunhands.mdl", nullvector, player_action);
}


Is there something wrong with my approach? How can I properly get the rockets to hit the center of the screen?