The way to do this is really cheesy just because it abuses the store buying / selling mechanic. What you'll want is a gun with a very high price, it can be anything. A magnum, a sniper, whatever may be as long as it has a high selling price:
















Next, you'll want to sell it. Then you'll go to the store and buy it again as the game allows you to buy parts & weapons you sold back again:
















Finally, Progress:










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V





You can stack this as much as you want to make progress go quicker, but this will skip the mindless grind of just buying things in the shop until you do get the challenge. This may be obvious to some, but hopefully it will be known to others! :)