The M590 is a shotgun which means spray damage and no accuracy really needed (just point in the general direction). Any shot chest and up does (being generous to the one getting shot) 25% damage. The shotgun is firing multiple pellets. If you fire at the chest of a guy... lets say 5 pellets are from every shot.. so 5*25%= 125%. So obviously, it's going to be a one shot kill. The range on the weapon is a bit random. Close to mid range its dead on, but I have gotten random kills from mid to long range. The max range seems to be somewhere around 40 meteres (tested with the distance scope).
Anyways, getting to the point of the OP, there is no such thing as a noob weapon. I've seen people who are absolutely terrible with it. I often use the shotgun as my primary weapon when i'm saving my money. It gets the job done. Doesn't mean its a noob weapon. Not every person who starts playing CA can be "devastating" with it.
Same things goes with the LAW. People often call it a noob weapon and yet it doesn't cause any real havoc on the battlefield. Yes it has spray damage, but its only 3 rockets and it's slow as hell. You can even shoot the rocket in midair.
If guns which are easy for people to pick up and cause lots of damage are considered "noob" weapons, then why isn't the frag considered a noob ball or something. People just complain about things that kill them and since they can't counter it effectively or let it frustrate them, then they consider it to be a "noob weapon" to make themselves feel better.