Doesn't work in the original game (main storyline not addons)

@Almorium If you've tried starting the game from the very beginning, addons will be disabled on the level transition between being captured in the elevator to being put into the Combine holding vehicle. Only Valve will be able to fix this on their end. It's entirely possible to start the game on the 2nd chapter (after the train ride) and play through the whole campaign with this mod enabled.

@McMessenger thanks for the reply, i'll do that next time I play because I prefer your pistol to the original.