***Easiest Place to Rank Weapons Up FAST!!! (DLC-5 UPDATE)***
As of DLC-5 the best way to boost your weapon skill has got a whole hell of a lot easier. Get into a Major Dark Matter Arkfall with whatever weapon you want to max out.
I recommend you use a SMG, LMG, or AR. They go pretty damn fast.
When you get to the end with the boss fight with the Monolith, activate one of the glowing green pillars, then attack the Monolith. The green pillars make your attack do something like 10X more damage and you'll rack up points super quick.
DLC-5 made this easier because you accrue points towards mastery regardless if the weapon is mastered or not. So no more switching out guns. Just use the same one.
You can rank up one weapon multiple times in one fight using this technique. If you get bored fighting the Monolith you can rack up some fast points fighting the Volge Warmaster as well, but it in nowhere near as fast as the Monolith.