I feel that it should be a separate bar that could eat into the main stamina bar as the player forces their character.
When it eats into the stamina bar the weapon will jiggle a slight bit, getting worse until it is depleted. When it is depleted the weapon can only fire from the hip with less accuracy. The aim stamina bar will refill fairly quickly though.
Main stamina will replenish slower ( and a little faster with perk)
The bar would deplete faster when carrying an LMG while firing in a standing position. A little less when crouching and deplete very little when prone.
(Prone shooting would make more sense if the mounting mechanic worked properly)
Same thing goes for anti tank weapons
Smgs,semi automatic weapons, will be affected the same way but with less drain on the aiming stamina bar.
Weapons with granade launchers will drain a bit more stamina than bolt actions but it will be less than a semi auto
Assault rifles will behave depending on firing mode.
(Semi auto it will behave like a semi auto, full auto and it will behave like an smg but with more drain depending on duration of shooting. Not as much as an LMG