I'm experiencing random weapon changes.
While moving, running or firing I've seen the weapon switch to another equipped weapon. Sometimes it will promptly switch again.
This is particularly annoying when it unexpectedly switches to a Rocket Launcher just as I hit fire on a target less than a meter in front of me.
I don't think I've seen it happen when standing still.
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If you're using a controller, then that would sound like to me that it could be an issue with your controller. Maybe something got in it and is messing with the wiring or something. I was having an issue where sometimes when I start aiming down sights with my Xbox controller, the aiming trigger would occasionally also act as the firing trigger, which got pretty annoying. After a reevaluation of my controller and beating it up against a table, I have come to the conclusion that something got caught behind my aiming trigger because it temporarily fixed every time I did that. I would not advise being like me and potentially breaking your controller though. Try and be nice to the hardware.