pro pipe is when the grenade launcher attachment is used tactically, ie. in the situation where when an enemy is wounded but escapes to cover behind a wall or around a corner and other enemies are advancing, so the grenade launcher is used in an intelligent way where no other option was available.
player's buddy: "nice use of the pro pipe! but, you're a fucking noob for using the M16."
ONLY CoD FAGS WOULD CALL IT NOOB-TUBE.
Awesome person- That's Pro-Pipe to you motherfucka.
Case 1- If someone uses a grenade launcher because they cannot kill otherwise- you would be correct in the assumption that it is a "noob tube".
Case 2- If someone uses the launcher because he/she is bored of the guns, and prefers to find more inventive ways to kill people (i.e. curving the ordinance through window frames, arcing at the right angles, hearing footsteps and killing someone who is around the corner, shooting a dud at close range to hit them in the stomach so they make that satisfying "ooof" sound) then that is not a "noob tube". Case 2 is a PRO PIPE.
In case 1- the user is easy enough to counter. Wait until he/she is reloading or force them into close quarters to truly test their aim. They only have one chance to dud shot you.
In case 2- it does not matter. Guns, knives, grenades- the user would have killed you anyways. You are their play things.
Generally- people have to have something to whine about on Xbox Live, PSN, or PC. Noob tube can be considered a derogatory term to those skilled enough to know the difference between cheesing out the grenade launcher and appreciating it's useful (yet slightly different from a gun) ballistic tendencies.