I was so excited to find it in Vault 34, but for some resons it keeps firing single shots only, so it is not as damaging and useful as I thought. Is there something missing with my character not able to use it....?

Rapid semi-auto high-precision rifle. If you want a full-auto rifle, get the assault carbine.

Basically, it's a super-service rifle with a scope. It has a much narrower spread cone, and has an inherently faster ROF.

Semi Automatic weapons are one shot per trigger pull. It's just that you can pull the trigger really really fast

http://fallout.wikia.com/wiki/All_American

In the long run, it's better as any time I have used a fully auto weapon, all I do is use up more bullets than I need to.

Against cazadores, there is no such thing as "more bullets than I need to". Consequently, full-auto guns are second only to impact-detonated explosives in caza-killing effectiveness.