I am level 48 now with 1day 1hour game time, That was in my swine flu recovery stage, now i am back at work.
To level up quickly to unlock guns etc, its best to stick with one gun and complete the challenges, I have been using the M4A1 starting with the noobtube (grenade launcher) getting my kills, then you unlock the red dot sight, holographic, thermal, acog, FMJ.. etc..
You get 5000,10000xp for unlocking all attachments
You get 10000xp for getting 500,750,1000 kills
So you can get from 15000 to 20000xp in a game, it does boost you up quickly.
I am 40 headshots away from completing the 500 headshot challenge for another 10000xp which completes the gun.
Then i can move on to the FAMAS or SCAR-H.
I am online alot so if anyone is on the 360
GamerTag: II HARMAN XD