Using F to select both the rate of fire and the barrel to use on a gun gets pretty hectic.

In a firefight, it can be a little tricky to tell which barrel you'll fire at a glance, and switching through both barrels and fire modes is a bit finicky.

Perhaps, like Ctrl+G chooses grenades, Ctrl+F could select the barrel to fire? 'F' would change the fire rate of that barrel as normal.