I personally found this achievement's description slightly misleading. You don't have to find 50k scrap dropped by slain enemies, you just need to get your scrap balance above 50k. This might sound harder, but scrap from scrapping weapons at a workbench counts. So either stash every weapon you find in the locker and scrap them all at once, or scrap them at the work bench but don't spend it. (You can scrap any unequipped weapons at the workbench.) Either way, make sure you pick up every gun you find.
*** Spoiler - click to reveal ***Highly recommend exploring Jupiter Enterprises following the There Must Be Something Left quest if you are looking for rare guns. After completing the quest I had 2 purple primaries. Went back in on the 5th floor and fought my way up. By the time I got to the safe space on the 13th floor, I had nine primaries and 3 secondaries. There is nothing too difficult in the ascent either, you can use the many doors as choke-points and mow down the baddies as they stream at you single file.
You only need 4 Rarity 5 (purple) primary weapons for this, as they scrap for 15k a piece, netting you 60k scrap and this achievement.