To fully upgrade the M4
you will need $1,625,000
(for all 5 trophy-releated weapons):
Simply upgrade any weapon to the max, don't forget to confirm with
, then go ahead and complete the mission. As soon as the trophy shows up, do not save your game
, exit out of the game and reload your save, you need to go all the way back on the main-menu screen and close career. You will then have the funds to upgrade another weapon fully. Repeat this process for all 5 weapons (Silverballers, SMG Tactical, SP12 Shotgun, M4 Rifle and the W2000 Sniper) in order to achieve all of the upgrade related trophies.
Hope this helps!