A fully automatic FAL
A crossbow. A fucking crossbow. It would have a slightly straighter tragectory than the noobtube, be a one hit kill, and totally silent. You don't even get the dead teamate symbol onscreen. This would be a secondary weapon, and you'd carry five bolts. Reload would be around two seconds (almost three). Enemies would clutch the wound and fall to the ground, it would stay in the wound. If it struck through a wall it could hold them up, if it hit someone who was near a wall, it could impale them to it.
Attachments would be an ACOG sight, an explosive tip (smaller blast radius than grenade launcher). And a grappling hook.
If you melee attacked with it equipped, you would stab your opponent with the loaded bolt. Only if you weren't reloading/still had ammo, naturally.