You should be able to get this pretty early in the game. During Leon's campaign one of the very first puzzles you come across will be in the West Office. You will find a note on your desk explaining that you need to crack the two combination locks using the first letters of the other officers' names.
Once opened the desk will contain a high-capacity magazine which can be combined with Leon's pistol.
I believe the puzzle and codes are the same for Claire and she gets a speed reloader for her revolver.
As far as I know this is the earliest place in the game to get this achievement, at least without cheating.