I'm pretty sure an Assault Cannon doesn't fit in a Standard weapon mount...
*sighs* It doesn't have to.
A Dodge Scoot has Body 4.
A Heavy Weapon Mount is 4 Mod Slots as of Rigger 5.
A Vehicle with a Body attribute of 4 gets 4 Mod Slots in each category, including Weapons.
Thus, RAW means a vehicle with a Body of 4 can technically install a heavy weapon mount.
And this is why I don't care about RAW as much as some, because there's no way I'd allow something the size of an assault cannon to be mounted on a moped...
By highest order you mean utmost importance right?
Spot on

Absolutely. It's why we have GMs. I'm the same way with really any equipment moding rules in Shadowrun. I go more by what "makes sense" than RAW because, occasionally, RAW is very silly.
Indeed. Hear hear.