40% ammo is pretty low so I don't think it would be awful.
Personally, I'd just have it automatically reload whenever you reload and not have them be separate loading sequences. That much extra ammo for Alt-Fire modes is still pretty solid for a T2 Item combined with the Alt-fire +Fire rate.
Alternatively, you could give it a second passive that allows you to fire your Alt Fire and Primary Fire concurrently. Just reduce the Bonus Alt-Fire ammo by like, 10% and remove the +Fire rate and I think you actually have a really cool item that would be pretty handy for heroes with Alt-fire modes who want to go gun builds.