In my game (the one in my signature) I also have lots of weapons and you can shoot left and right. The way I did it is that the player has a variable called "direction", and when he faces right direction is 1, and left is -1. Bullets also have this variable, and it sets if it is mirrored or not
The weapons all move based on math, so no behaviors. And instead of setting its angle, I use a variable to tell the angle of the weapon movement.
Example:
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