Non-magical firearms are allowed; in fact they're listed (as "Guns") under several characters' allowable equipment! But understand that depending on the weapon, some performance nerfs may be in order. There's a difference between an M1911 and a GAU-8, after all.

Given that there are similar degrees of 'nonstandard', atypical weapons may need to either be redefined in a more setting-compliant manner (for example, a DnD Half-Orc Barbarian using their former party member's petrified body could be defined as a Hammer)

Additionally, a character presently needs their current class to have proficiency with a given weapon in order to use that weapon.