Basically, what this glitch does is it will get you any gun you want (even classified). Obviously you need to be level 4 so you have create a class. And this works for pro perks too.

Instructions:
1) Invite one of your friends to a party and game.
2) Backout once he's in.
3) Go to system link. (lan party for PS3)
4) Highlight over the name of the gun, or perk you want (make sure it says pro)
5) Have your friend invite you to his game.
6) While still highlighted, press Xbox button and join that way.
7) The screen will come back after it says joining session, so press A then up, then A again, then up again. To buy and equip.
Note: You must do this step 7 quickly because after it says joining session and it goes away, there is very little time.
8) You will then be in your friends lobby, and the gun or perk you selected will be in the class you highlighted over in system link.
Note: Copy your class to every class, like if you bought tactical mask pro, copy it, and you will have it for every class.
This does work. I have confirmed it.