Funko, Shigu, Eru and Ichiroku seem to be typical school girls on the surface, but they're actually powerful, dangerous guns that must be treated with care and respect! Together, they attend Seishou Academy, a special place where not only are all of the students guns, the teachers are as well. That is, except for one new educator who arrives, unaware of the crazy, dangerous times that lie ahead!
Each of these mangas tells the tale of a group of deadly weapons which take the form of human girls/women.
If you want more girl-weapon transformations you could try reading Kandachime. It has has swords take form of human girls (rather than guns like it was in Upotte!!) and it's probably better than Upotte!! in terms of story, characters and art.
Looks like it's the season for making manga targeted at military otakus (or trying to turn those people into lolicons). Like Upotte!!, Girls & Panzer revolves around moe schoolgirls handling military equipment, except instead of girls with guns we get girls with tanks. The tank trivia is also there (just like gun trivia could be found in Upotte!!). At least Girls & Panzer doesn't have fanservice...