In Payday 2
, the contents of your Inventory and your chosen Skills are saved locally in the "Player progression" file on your 360's hard drive. As such, if you copy the "Player progression" file to another source of memory for your 360, such as a flash drive, and purchase additional weapons or skills, those changes will only synchronize with the specific "Player progression" file from the source of memory that you have selected.
What does this mean? All of the achievements relating to purchasing skills or adding to your inventory can be gained by making local copies of your "Player progression" file on an alternate source of memory than your primary hard drive, and using this copied file to make any desired changes to your selection of inventory and skills, with no resulting change in your primary "Player progression" file.
To unlock the "Fully Loaded" achievement, you can still purchase however many of the introductory $12,000 weapons, but now you can spend your money with a local copy of the "Player progression" file on another source of memory, rather than having to spend that extra money on your primary "Player progression" file.
Credit for this solution goes to GatorFist