There are no noob weapons
Each gun has its pros and cons. Shotty and smgs are for close combat. you cant snipe with shotty. you cock it every shot and reload time is horrid. yes p90 shoots 50 bullets b4 reload but again reload is horrid. if you have an assault rifle with extended clip and silencer and scope there is no reason for these guns to get best of you because your gun is more dynamic, usable in close and medium range combat. I keep a shotty for when my clip is empty. I don't however run around just using shotty. I think the sniper rifle "noob cannon"
should also be mentioned. you can no scope with it but its not easy. so it too has pros and cons.
I guess a slingshot or BB gun could be considered a noob weapon.
If you getting pwned you should change your tactics.