Most likely a rumour, arms would be only allowed on board let alone airside and handed to the crew member, after it has been made safe.
Being "made safe" means, carrying out the following procedure:
Mags removed.
Bullet ejected from the firing chamber.
If the above was done.
Arm then fired safely via the trigger to ensure nothing is inside, as it should.
Then trigger locked.
After which, arm and mag separately handed to the Captain for safe storage, before boarding.