Note: You can only go after these trophies after you earn Tier 4 Kruger Schmidt .
All W2000 Sniper upgrades cost approximately $1,500,000.
If you fully upgrade a weapon in the Kruger Schmidt shop and the trophy doesn't unlock, do not panic! The customization trophies do not pop once you fully upgrade a weapon but after you beat a level with said upgraded weapon. Upgrading all weapons will cost you a total of approximately $7,500,000. It will require a ton of farming, so if you're not up for grinding some missions over and over again, go for the money trick. The best levels to do this in have to be A New Life and Curtains Down as they are the shortest missions in the game(Not to mention easiest if you merely gun down your targets and run for it.).
Select either of those missions from the menu and once you're taken to the shop, fully upgrade your weapon of choice. Start the mission and finish it however you please. The trophy should unlock on the results screen. Now, DO NOT SAVE. Quit the game by holding down the home button and reset it. You will have your trophy for fully customizing ______ weapon along with all of your money intact. Sadly, you will lose your latest upgrades. Continue to repeat the process with all weapons until you have all customization trophies.