Thank you very much for picking it up !
Let's hope he won't reproduce WWI weaponry... Or it's gonna be gruesome.
I hope they don't for the sake of aesthetics. Colonial era warfare and black powder are so underrepresented in media. Though with how much attention to detail is given, I'd wager the mangaka is an enthusiast of this era. Starting with this premise then ditching it for world wars stuff would be silly.
With regards to the MC, he doesn't come across like a typical isekai protagonist that flexes with his "superior modern knowledge." He introduced rifling and the minnie ball out of necessity to compensate for shorter barrel lengths. He hesitated and doubted whether he should've done that. Everything else he's done so far is to compensate for his troop's weaknesses and give them as much of an edge in survivability as possible.
If there's already muskets being used as regular infantry weapons, it's just a matter of time, regardless of whether MC does something or not
While it is a matter of time, I doubt it will happen in MC's lifetime. Without machining, any fancy invention he recreates will just be novelties: hell, we have examples of cartridge using guns as far back as the 1600s, but it never caught on in the military until the late 1800s. Rifling and smoothbore both have their places, and while rifling is a considerable advantage to accuracy and range, it was rarer due to expense. Understand that any military in history are a bunch of cheapasses and will cut corners whenever possible. "Military grade" = built by the lowest bidder.
In terms of WW1 weapons, true reciprocating cartridge firing weapons took a few centuries to crop up after the advent of rifling. Not to mention that rifling itself wouldn't be out of place in the represented time period. The next big invention would be the percussion cap, as that gets rid of the need to prime the pan and makes matchlocks and flintlocks obsolete. But MC won't figure out how to make them off of cursory knowledge, he'd have to know what mercury fulminate is and how to make it.
Thank you for listening to my TED talk.