The Empire's focus on mana-tech has led to the severe underdevelopment of conventional firearms. Thus, if you want your character to have access to any Martial or Advanced firearm, this choice needs to be explained. For example, they are genius tinkerers, who built all kinds of weaponry since childhood, or they are crazy gun-masters, who spent their youth reading books about firearms.

However, at level 7, any character can take a feat granting proficiency with Advanced mana-tech weapons (mana-gonnes) manufactured by the Empire’s schematics.
At level 11, a similar feat becomes available for proficiency with M.E.C.H (Mana-Enhanced Combat Harness).