I've been my whole today gameplay throwing my M4 without knowing the reason.
I decided to delete the assignment of throwing item, deleting the "G" assignment, nothing solved.
Later i found that this bug is started by pressing the keys "Shift" plus "+"
So as i have my turbo run on "shift", when i zoom with "+" i automatically throw my weapon away.
This is a very big issue, i've been looking for my M4 more than thinking in the actual game.