How do you buff your own gun?

same way you buff yourself as a medic. i don't remember what the default is, but in the controls there's a "Buff Self" action

You click and hold the left anolog stick

Oddly, LB on x-box is sprint/smart, and clicking the right joystick buffs self, but you HAVE to hold down, not click once.

Yea i change my sprint to the stick and self buffs to LB. Idk why the devs told us not too, it's not hard to hold down the stick while using smart.