Little hard for me to explain. When scrolling through the weapons(v1.31) you have to hit fire once you have the weapon selected from the list. Can cause problems when in the middle of a firefight & need to get to another weapon quick.
I remembered that Half-Life/TFC/CS defaulted to similar system & you could change settings so that the weapons swapped instantly w/o having to hit fire button, just rolling the scroll wheel would change the weapon/item you were carrying.
That clear it up?
Miah