As you can see in this post: Double-barrel Shotgun , the designer said that.
The point is you get an extra shot, so if you shoot, and roll a blast, your weapon does not empty (you don’t flip the icon) and you can shoot again (maybe I should have said “between” activations), and even if you shoot again with your second action and roll another blast, it does not empty.
So there is no doubt about shooting if you have it previously loaded, right? Maybe I did not understand the question well 
You can shoot it every time you have it previously loaded, it is a rifle type weapon, so no problem with that… is not the same as a Minigun or heavy weapons.
2: There are some icons wich don’t appear in the rulebook, like that one or the Knife icon… Hair trigger’s icon means an Action Point Use Icon for Shooting actions (in this case, it would be Shooting actions from that modded pistol/rifle only), so the model could use the APUI from the modded weapon for a Shooting action with an AP using this modded weapon.