In experimental 1.05 there is a key binding called "Heavy Melee Attack" or maybe it is just "Heavy Attack" (sorry I'm away from my gaming computer right now).
I mapped this to the down arrow key. I expected this to be the key to hold down to give me a stronger but slower melee attack when punching for example. But it seems like this key is actually the key that swings your firearm ("weapon bash") when you have a raised firearm in your hands.
It turns out that I can do a "heavy melee attack" but it happens when I hold down my sprint modifier key while punching.
So either this is a bug or maybe the "Heavy Melee Attack" key binding has the wrong label. Maybe it should be named "Weapon Melee Attack" or something similar.