To believe that Naruto is a fictional universe that's still very primitive is honestly a bit silly. To say that there's no guns in a world that has laptops and cell phones doesn't make sense (unless it was a peaceful world where guns aren't needed, be we know that's not true lol).
The likely reason where there are no guns (excepts for the one that's early on the series) is there really isn't a need for guns by the average ninja. Do you really need something that can only be used a few times before being needed to be reloaded on top of necessary maintenance when you can shoot fire out of your mouth or make the earth come up and crush people?
No, you probably wouldn't need a gun then. But why does the shopkeeper have a gun? Probably because he can't do those things and it's probably the only way he can protect his shop. But that's all speculation and time to go back to the topic at hand. And I don't see why it can't have a similar mechanism to how real firearms work, considering it's not really that advanced. Hell, I can build a shotgun at home with nothing more than some basic supplies. I could build an even smaller, hand-held one with not much other than some pipes and a nail.
At the end of the day, the use of the guns in question are dictated by Deathstroke. It's his decision on whether or not he'll use them in a fair way, or if he'll be popping off headshots from across the village left and right. My bet's on the former. The combination of elemental, physical, and sealing rounds that's at hand leads to a very interesting kind of gameplay.
And for the most part, the complaints can be summed up as 'Waah, my character can't dodge/sense it' or 'It's too fast'. But in this game where everyone's as fast as Minato and can sense everything better than a Sage with Mind's Eye, is it really 'unavoidable/undetectable'?
I can understand the apprehension about the sniper rifle in particular, but I doubt he will be using it for only headshots. More likely, he'll be using it for either body/limb shots for harming/disabling or be using it to apply long distance sealing for support. And I think that's something that people haven't really done before. Most sealing usually requires physical contact from what I've seen (or Kamui). So because he's breaking the norm and trying something new and interesting, my vote's all for it.