As there has been much discussion on this issue, we wanted to provide an update. The original M4A1 was an unintentinally overpowered weapon, and was causing a severe imbalance in the game.
The decision to change the item drops rather than nerfing the existing M4A1's across the board was reflective of the amount of time and effort many people have put into obtaining them. We had to fix the on-going imbalance, but we did not and do not want to invalidate the effort that our players have already invested in improving their characters.
Going foward the drop will now be the Rusty M4A1.
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