You can detail the inner mechanics and technical workings of all the firearms from the post 1900s in however excruciating detail that you want, but that still wouldn't be relevant to argument here. The manga never goes into all that. As far as I can tell, it provides very limited information that can be corroborated with real world rifles from the period. As an example,
@Kend0 mentions the Dreyse Needle-Fire rifles, which is apparently classified as a bolt action rifle (unless you tell me this is wrong.) The manga makes no indication, again
textually, whether the rifles Weißen uses are breechloaded, smokeless or whatever have you. The few capabilities it describes of these rifles are seemingly in alignment, for example, with those of the Dreyse rifle. While the manga mentions a range of 400 m, Baltzar acknowledges in a later chapter that's far too long for practical range, as you yourself say. All that said, the Dreyse rifle was apparently designed in 1836. Rate of fire of 6–12 rounds per minute. Effective range of 200 m, maximum range of 527 m (as per Wikipedia. If you have issues with the source, that's another argument.) Manga details Weißen rifles capabilites as Rate of fire 12 per minute, effective range of 400m, potentially lower. This for rifles used around 1868. If you ask me, that lines up pretty darn well. I'm sure there are actual real world rifles used in this period that probably line up even better. Now, the art it provides for said rifles might be wrong, but I've already covered that. If there is some part I'm missing here that actually gives more details about these rifles, mention it.
The manga gives
copious other examples for saying that Weißen is around the 1860s in terms of technology. I've already detailed all this. If they are using tech from the 1930s as you seem to say they are, surely the other technology Weißen uses must match up with that timeline. And not
just the rifles. The development between pre-1850s muskets to 1930s rifles can't happen in a vacuum independent of all other technology being used. While this is obviously not directly related to firearms, and not the best example to support my argument either, take the Woodburytype used in chapter 11. By the early 1900s woodburrytype was phased out by halftone printing. There are other examples like this. Gattling guns are still not a thing in-universe. Volley gun's have yet to see standard field use. All such decidedly puts the techonological level
not post 1900s.
If you're just going to ignore all that, and say because Weißen seemingly uses rifles from the 1930s (which I still don't see an actual justified basis for) or WW2 (when even WW1's trench warfare isn't a thing yet in-universe, so idk how you're jumping so far ahead), and because of this there is a techonological gap of almost a 100 years between Baselland and Weißen or neighbours, there is no argument to be had there. The
only issue I can see here is the rifle. Which is possibly just the author mistakenly using a wrong reference art.
Equally, if we then establish that okay, Weißen is around the 1860s, and Baselland is around the 1840s, and such a gap is still too egregious and Baselland should have been long since annexed, despite the fact that I've already mentioned Weißen's other military campaigns occupying it's attention and Baselland itself wanting an alliance with Weißen, then I'm not going to argue that.