So, Haven't really been on NGU For a while. So Idk if this has already been posted, but decided to contribute after a while.
Again Don't know if this has already been posted, if so, don't flame please<3
basically, you know the challenges that give 10,000 EXP right? such as the veteran challenges, well there is a way to double that EXP, a total of maybe 3 or 4 times. or just level a gun up quickly so without further adieu, the "Glitch"
Basically get a gun that you almost have enough kills to get 10,000, such as a 500 kill challenge. once you get to 499 or close to the 500, to where you know you'll hit the 500 in the next game, attach that gun, and select one of the "2 attachment pistols" like the .44 magnums, the DE, with a class with SoH on, and when you get the 500 kill, quickly switch to the pistol, and then the 10,000 exp will go to your secondary and give that the 10,000 giving it the profiency which is 10,000 exp.
tl ; DR
1)Get 499/500 kills with one gun
2)Equip 2 attachment pistol, and SoH
3)Get kill with gun with 499/500 kills, get 500 kills, earn 10,000 exp
4)Right when you get the kill, and earn 10,000 EXP, quickly switch to pistol
5)You will earn another 10,000. A total of 20,000 EXP.
Note: The 10,000 EXP from 500 goes to pistol, not primary gun, so you're primary gun WILL NOT rank.
so yeah, Hope this has helped.
and in case you guys forgot, after a game, and you see match bonus, hammer that X button on xbox, or  button on PS3 for extra match bonus.
Again, I don't know if this has already been posted, and if it has, I'll get a mod to close it.
Don't forget to hit the thanks button
5/05/2012 Update, with Proof.