It is found under subparagraph 515(6)(viii). Actually there are two reverse onuses that link to firearms. Subparagraph 515(6)(vii) says, “an offence under section 244 or 244.2...that is alleged to have been committed with a firearm”.
Then, under subparagraph 515(6)(viii), it's quite broad. It's any offence that's “alleged to involve, or whose subject-matter is alleged to be, a firearm...while the accused [is already] under a prohibition order”, as defined under subsection 84(1).
Subsection 515(5) deals with reverse onus as well.