Zeroing is a lot more than just velocity, it would have to be based on what round is currently in the gun and then the zero would have to be updated every time you fire which would look janky and wouldn't be realistic. Feel free to make suggestions in this thread or in the comments!
You missunterstood.
The rifle shouldn't be rezeroed during the raid, just properly zeroed at the start of the raid, based on its current configuration.
Alternatively it could be a function of the shooting range, where a player takes his rifle to properly zero it.