- at least 25k
- completing stages ''The Haunting of Nevada House'' and ''Slaughter Time''.

This exploit is done by using workbench. to do it you need to simply buy on of the LMGs from the black market (it can be PKN, Nextek 6000 or AN-2500).
Then, go to the workbench and give it this one of these sets of attachments:
(If you are later in the game you can also add 150 drum magazine for better prices).
After upgrading, simply sell it and do it all over again.

Also dont correct me if i made mistake in english. i have no respect for this language.