Combination weapons have been tried in real life, they were generally impractical. Expensive to build, able to fire only one bullet with poor accuracy, and too heavy to be used as well as melee weapons. You basically got one weapon that didn't work well at range or melee. Bayonets' are the only really effective form, thus the reason they are the only ones used in real combat, but I assume you want something more interesting then that.
I consider combined weapons hard to justify, but I can come up with a few possible justifications. I'm not convinced any of them are enough to justify combined guns, but perhaps they could work when used in combination with each other and/or other answers (I honestly really like GS' answer).
Magic covereth for a multitude of sins. You can make up any kind of rules you want for a magic system, it's entirely possible to come up with some that allow magically made combined weapons that otherwise wouldn't be practical. I won't elaborate too much on this though since I get the impression this is a low magic world your envisioning.
The main problem with combined guns is that guns are heavy, and if you try to make them lighter the barrel warps from the heat of the gunpowder. If, however, there was some super metal, one that is surprisingly light and, most importantly, can stand up to the high heat from a gunpowder explosions without breaking or warping without needing to be as thick or heavy as guns otherwise are you could better justify fitting a barrel into an otherwise normal melee weapon without the melee weapon suffering as much from the combination. There are still practical issues with reloading such a weapon without accidentally cutting your foot off with your swordgun, but the right material could make for a more viable option.
Since you already have meteors striking earth, and super powerful 'sky metal' is a standard trope in fiction I'd suggest if you went this route the material you need actually came from the same asteroids as your fantasy horrors. That would suggest mining them for metal may be a bit hard, making such weapons potentially expensive. That could work though considering another option...
Swords use to be carried by generals long after the odds of needing a sword in combat were slim. The swords weren't there because they were a practical and required combat weapon, they were there as a status symbol. They said "I'm rich and important enough to be able to afford my own sword, this is why you should trust me as your leader.'
Your combined weapons could be similar, though I'd probably go in reverse to make having a mythical gun, mostly lost technology, be the status symbol not the sword part. The rich commanders carry their combined weapon less for it's practical use in combat and more to impress those they are trying to lead and as a symbol of their worth as 'leaders'.
Otherwise impractical guns could make sense if legends and myths about them had instilled such a fear that your enemy is afraid to charge them or otherwise allows their fear of the weapon to prevent them from fighting as well as they otherwise would. Even if the weapon logically isn't otherwise as practical in a fight as a traditional melee weapon the moral advantage gained by having 'legendary' weapons could more then makeup for their otherwise impractical design.
So the weapon I'm envisioning would be primarily melee weapons, not long range ones. They would be able to shoot one shot, maybe two, but the reload times would be far too impractical to try in battle, it's a fire and forget thing. The focus is on making them viable melee weapons, not practical guns. They may have terrible accuracy and/or range due to similar sacrifices. The gun parts are in a sense an afterthought, the goal is to make something that works almost as well as a normal melee weapon but can still fire a bullet.
The main reason for this is because the enemy fears them. You don't really care if you even land a kill when you fire the weapon, the real victory is a how the sudden coordinated firing of dozens of feared weapons on the battlefield affects your enemies moral.