It is a gun, it functions as a gun; you use it to shoot projectiles at targets. You just can't aim it. I'm hung up on the fact that it's a gun, because this is a recurring issue in the Souls games. In Dark Souls it already ticked me off that you couldn't manually aim spells, you were depended on the lock-on, which has a margin for error, and everytime that margin occured I was out a spell. This was mitigated by the fact that spells would refill at bonfires, but it was still annoying. Then we had the bow which you could aim, but it was an aim so bad you avoided it like the plague. But eh, Fromsoft might still be a bit fresh to this whole action RPG genre, I'm sure they'll learn eventually.You're really hung up on the fact that it's a gun. It's not really a gun, it's a function. The function is to parry and stun enemies. To that end it could have been anything. Instead of having you fire a gun they could have had you throwing a brick, shooting off a roman candle, using a spring-loaded boxing glove, whatever. They chose a gun because a gun is a cool thing for it to be, not because they wanted the function of a gun.
To explain this with another example: Halo has a weapon, the energy sword. People aren't complaining that the game doesn't let you have big cool sword fights with the energy sword with blocking and parrying and ripostes, because that's not the function of the energy sword. The function of the energy sword is to be a close range power weapon that can one hit kill. It's just the Covenant equivalent of the shotgun in terms of function. It wouldn't matter if it was a sword, an energy shotgun, a short range heat ray, or something else. It exists to be a close range power weapon, it doesn't exist to be a sword, despite being a sword.
Out comes Bloodborne which has a straight-up gun - no spells, no bow - a gun... and you still can't bloody aim your projectiles. I've been on this spiel before and I wouldn't have as much of an issue with the mechanic as is if your bullets refilled at lanterns, which they only do if you have a supply in your stash. And while you do pick up ammo as you go and probably collect a nice amount so as not to run out most of the time, running out is definitely a risk, and you'll probably end up farming for them.
The gun was marketed as a replacement for the shield, the game even as a gag shield as a joke, as in 'why would you use this now that you have a gun', but the shield didn't use consumeables unlike the gun. The shield needed be repaired with the use of souls, so in a sense it did use consumeables, but since souls are the currency in the game you always picked some up. If the gun could use souls to convert to ammo, at a vendor or the lantern or whatever, I probably wouldn't have as much of a problem with it neither.
It comes down to the gun just not being fun to use. You don't feel empowered or skilled when using it, you just feel awkward. It's annoying and cumbersome, yet also your main mechanic for staggering enemies.