Shot particles dont show up?

I have particles running infront of my gun, they are disabled in Start but when i click they should show, and they dont? Any Ideas

Here is my script:

#pragma strict

var GunFlashP : GameObject;

function Start () 
{
	GunFlashP.SetActive(false);
}

function Update () 
{
	if (Input.GetMouseButtonDown(0))
	{
		GunFlashP.SetActive(true);
	}
	
	else
	{
		GunFlashP.SetActive(false);
	}
}

What’s happening currently is when your mouse button goes down the gameobject is activated in that frame but in next frame since it does not receive the mouse down it will deactivate the gameobject and since the gameobject is active for one frame you are not able to see the particle system.

So you want your Update method to only activate the gameobject on mousedown.

 function Update () 
 {
     if (Input.GetMouseButtonDown(0))
     {
         GunFlashP.SetActive(true);
     }
 }

The else condition is not required here because you have already deactivated the gameobject in Start().

OR

You want to show the particle system for the time mouse button is pressed down:

 function Update () 
 {
     if (Input.GetMouseButton(0))
     {
         GunFlashP.SetActive(true);
     }
 }