Maybe whoever stored the gun away thought like that, so the first one up is empty.
The scenario I'd dreamed up was that it's a personal firearm stored away by a previous owner decades earlier, discovered by the protagonist during a moment of crisis (the existence of the firearm was already known - but its whereabouts was not up until this scene) and grabbed out of desperation for lack of anything better.
I preferred to have it that a couple rounds of ammo were 'bad' (just because of the age of the gun), but that it does eventually fire. I'm in a bit of a quandary though because, while a couple rounds of bad ammo is dramatic/suspenseful for storytelling purposes maybe it comes off as too unrealistic. So my other option was to have a couple of empty chambers, but now even that seems lame ...