Underbarrel/ads toggle fire mode

Here's the answer and idk why no one at dice did this. 

Why can't the game detect when you're using a weapon with an underbarrel attachment? 

So when you use the setting "Down on the D pad to switch to underbarrel attachment/ads toggle fire mode" you only have to ads to toggle fire mode while using a weapon with a underbarrel attachment. Then when you're using another weapon without the underbarrel, the game knows so, down on the D pad would automatically be used to toggle fire mode because there is no underbarrel launcher. 

Why is this such an issue that when I'm fighting people at long range using single fire, then switch positions, accidentally run into a person and I have to ads just to switch to auto but I'm already dead. Then I spawn, forget I'm still in single fire and die again.