The kind of funny bit, from firearms history, is that I'm pretty sure most of the bolt or lever actions we're used to know actually pre-exist at basic designs well before the metallurgy & manufacturing ability was available for cartridges. They knew what would make the most sense, it's just you couldn't yet produce them. That said, random farmers inventing precision measuring devices is definitely when to nope out.
Similar to the steam engine. James Watt knew what to do for a decade or two but couldn't get it to work until John Wilkinson's boring machine (to make cannons) allowed the precision necessary to seal the damn cylinders.
To elaborate beyond my glib comment earlier, the protagonist's an agriculture student, and revolutionises Japanese agriculture in the sengoku era. Convenietly brought seeds with her. Has trouble navigating past social customs. Is cute. So far, so good.
She also brought a book on weapons with her (how convenient...), and is pushed towards a military role. And it's here that the author clearly loses it.
If they'd approached local metalworkers and used foreknowledge to produce decent springs that could in turn be used to produce flintlocks instead of matchlocks, okay, I could've lived with that. Hell. Flintlocks were technically already a thing in Europe, just stupid expensive and relatively complex because springs are hard. Modernity rests on metal springs, but I digress.
But nope, it's bolt action rifles out of fucking nowhere.
You know, I wouldn't mind an isekai loosely modelled on Gonzalo Guerrero. Shipwrecked, enslaved by locals, impresses them and ends up becoming a thoroughly tattoo'd war chief and marrying the king's daughter. Hella impressive, one might even call it wish fulfillment, but it did happen.
But lacking the necessary economic basis or indeed, professional experience, Guerrero didn't end up throwing the Maya into the iron age and start building arquebus on his lonesome.