never going to happen.
ever this manga seems to be focusing on an actual exploration of what would happen if someone from modern day(with the help of some textbooks) actually went into the past. she's limited by her technology and what's feasible. note that they haven't even dug out rifles or repeating guns. they physically cannot build those. same for any sort of modern repeating crossbow or so on. think of it this way- the progression of technology isn't just know how, it's also infrastructure. you create machines that build more machines, which build even more machines. and that's for something as "simple" as a rifle
let's explore the process of making a rifle from scratch
first, you need a barrel, easy right? you already have techniques to do so? so how do you make it rifled then?
it's not easy. rifing is essentially one of those things you
need very good tools for. essentially you need a lathe, and a precise one at that. go look up the difficulty of making lathes without yanno... lathes. secondly, if you don't do it right, well
boom your gun just exploded. and so on. I think it'll be a fair while before we even see rifles. we almost certainly will never see anything "modern" in Shizuko's lifetime if the story sticks to it's take on realism
these aren't antitank rockets, they're closer to primitive artillery rockets. I don't know about the range, but at least according to this article, the chinese were using rockets since the 10th century, so by no means is this a stretch. the only think I'd question is the range, but you'd be suprised what can be done
https://en.wikipedia.org/wiki/Bo-hiya