One thing I have noticed a lot in this game is Pre-Mature nades and my gun misfiring!
Even if I am standing still when I press 4 for my nade and get ready to cook it, it just throws!
I also have a huge problem when running into a fight and start to hold me trigger down my gun doesn’t fire!
Is this because of sprint or crouch? And bottoms being pressed at the same time?
Could it be on my end with a bad mouse?
I’d say mouse too, and maybe lag/server lag under heavy load.
You can pretty much fire crouching, jumping, falling, I’ve not experienced your issue yet.
Regarding the nade, no offense, but is it possible you bound the quick nade ability instead of the slot?
I have the same issue, i thought is my mouse, you are not alone 
I blame the network for the Pre-Mature Nades and Misfires.
Please raise a bug about this in the bugs forum, that way it’ll get bugged and tracked on our end. I have seen a bit of this myself so it does need investigating.
I’ve gotten some delay in actions before, so maybe that’s the issue. Like your switching weapons while clicking and you end up misfiring because of it.
This is what I’ve seen several times and I’ve reported before.
Plus, if you use sprint toggle with the shotgun you’re prone to having that gun misfire so often you’re better off using hold to sprint.
I have experienced several times my shotgun not firing but I have no idea how to reproduce it. Could be random netlag, yeah.