I was missing a detail.
I use
Round Start Items Giver and at beginning of every round and the players receives a Magnum pistol (even the Prez).
This is great because the player chosen to be the Prez has a better gameplay and many servers have the infecteds overpowered (e.g. my servers).
So... two suggestions:
Donīt block the Prez to receive the Magnum in case of some adjustments (just let as is);
or
Create a cvar where is possible to choose 9mm or Magnun to the Prez.
Congrats again!