You might be thinking of Sakuracon's policy, which does state that the gun could never have been functional. Even if you disabled it, if at any time it could have fired, it's not allowed. It's incredibly draconian, and I really hope Kumoricon never goes down that route, but that's just my two cents.
Speaking of rules, are functional Nerf guns allowed if no darts are fired, or even brought? I've only heard policy on airsoft.
Also, I don't know what kind of guns you're looking for, OP, but gift shops in tourist towns or small theme parks like Enchanted Forest often have more realistic toy rifles and water pistols than you might be able to find in toy stores here. They'll almost definitely be less accurate than airsoft, but they'll also almost definitely be cheaper, so think about it if you're going on vacation.