[@Cleverbird] You're not totally wrong, though as I said these do happen to be aliens that have been attacking since those guns were (relatively speaking) new. I'll go on and spoil that we aren't only going to be dealing with the cannon fodder aliens that have assaulted humanity as there's clearly something more to it than just sending waves of drones to die. That said, even an early model 9mm pistol could easily kill the basic aliens that have nothing to their benefit past their large numbers. 

That said, comparing them to a musket's a tad unfair as combustion based kinetics don't really have much room to advance between now and then. Yes, non-kinetic or rail-based weapons are considerably better. Yes, Those weapons are definitely going to be a disadvantage later. For the same reason that flamethrowers exist even in super advanced Sci-fi universes, pistols like that could certainly still be effective. 

Actually, I'll give credit to Severance on this one where I normally wouldn't have in a similar situation. The different types of rounds imply that at the very least the ammunition is a bit more on the advanced side.

You're bringing up a legitimate concern Clever, but it's definitely not as bad as it might seem. Really as far as kinetic weapons go, the ammunition is more/less the only thing that's got room to improve (at least to that degree). It's why most sci-fi worlds that don't just go with lasers and plasma generally have kinetic weapons that are next to identical to modern ones. 

Oh, and flamethrowers. Some tech hits a point where advancements are minimal or kind of unnecessary, and so completely new fields of technology are birthed.

Of course, [@Severance] You could always make some sort of barrel attachment up to increase the velocity of the rounds in order to keep up with other weapons more seamlessly. It kinda seems like you want those weapons for more of an aesthetic purpose, so I'd say you can definitely have the best of both worlds.