1) Make sure your at least level 4(custom classes unlocked)
2) Get a friend to host a lobby for you on the Xbox live/Play Station Network screen.
3) Tell your friend to go to Private match and wait...
4) YOU go to create a class, Go to CUSTOM CLASS 1, and scroll down and select COPY.
5) Once your on the DESTINATION screen tell your friend to back out of the private match.
6) (you should be able to scroll in the background), go to CREATE A CLASS.
7) Select a Class that you least use and select PRIMARY, then go to SHOTGUNS Note: Its hidden behind the Destination screen under the sniper rifles.
Under SHOTGUNS go to the USAS 12(top shotgun), on proficiency click NONE, On Attachments click DONE and stay on the CAMO screen
9) On the CAMO creen press 'B' for xbox or '' on the play station ONCE<--
10) (You should be able to move on the destination screen)select a class you want damage on.
11) (you should be on the class you selected) now go to PRIMARY and select the weapon you want the damage on BUT WAIT!!)
12) Once you have selected your gun it should take you to it's PROFICIENCY
13) Press B on xbox or square on ps3 5 times, or until you see the proficiency screen again
14) Select damage and you're done
I suggest to use the MK14, it gets 1 or 2 shot kills most of the time!
I don't know who credit belongs to but I give full credit to him/her