Create a static shell for a bullet, without animations.
You can create more than one shell using
Code:
ent_createlayer(STRING* filename, var flags2, var layer)

Look for the function in the manual.
After creating the shell, give it a layer and position on the screen and animate it through script.
Add initial velocity upwards and right (for example) and then decrease the upwards velocity until a gravity effect is simulated.
If you want you can use world entities (ent_create) but then you have to make then spawn exactly at a specific point on the screen (possibly converting screen cordinates X,Y to world coordinates with a Z distance value).
This has the advantage that the shells can be left on the ground for the player to look at for a few seconds before fading away.

I hope this helps you.

PS.: vec_for_screen (convert screen coordinatess to world coordinatess) example
This isnt tested, but it should work. Write your own code for the animation after the comment line using c_move and a decreasing skill for z velocity...
Code:
function animate_shell()
{
    my.skill10 = 250; //lifespan
    while(my.skill10 > 0)
    {
      // animate it here
      // ...

      my.skill10 -= 1;
      wait(1);
    }
}

// In player action (if a mouse_left input)
    shell_spawn.x = 640; // Middle of resolution 1280x1024
    shell_spawn.y = 512;
    shell_spawn.z = 10; // Distance from screen
    vec_for_screen(shell_spawn.x,camera);
    ent_create("shell.mdl",shell_spawn.x,animate_shell);

Extensive Multiplayer tutorial:
http://mesetts.com/index.php?page=201