By Kazuma Kamachi and Motoi Fuyukawa. Released in Japan as "Toaru Kagaku no Railgun" by ASCII Media Works, serialization ongoing in the magazine Dengeki Daioh. Released in North America by Seven Seas.
Ah, franchises. In the beginning there was the light novel series Toaru Majutsu no Index (A Certain Magical Index), which started in 2004 and has shown no signs of stopping anytime soon. In turn, it spawned a series of side-story light novels, based on the breakout character Mikoto Misaka, called A Certain Scientific Railgun. Then a manga of Index started over in Square Enix's Shonen Gangan. Then a manga of Railgun (this series) started in Dengeki Daioh. Then the anime came out. Then the Railgun anime. Then the 2nd Index anime. Then the game...
It can get very confusing. Here in North America, light novels don't sell, so the books are not available. The anime has one season released by Geneon (now defunct), and the Railgun series due to come out soon. As for the manga, one would expect that you'd put out the parent series before you license the spinoff. However, as is quite common in Japan, they're owned by two different companies. And Yen Press, who deal with Square Enix a lot lately, have made no noises about licensing the Index manga. Seven Seas, though, does deal heavily with the company publishing the spinoff, and so that's what we have. Luckily, a) you see very little crossover between the two, beyond the hero, who is introduced to new readers; and b) Railgun has become more popular than Index anyway, so should do well here provided that fans of the franchise actually purchase a thing rather than merely gushing about how much of a fan of the series they are and then reading a scanlation online.
As for the manga itself, the supposed premise is that we're at a magic school (which is apparently so large it's the size of a city) where various powerful kids learn to control and harness their powers. Our here is a normal schlub of a guy who has no actual powerful skills but manages to win the hearts of over a dozen... no wait, wrong series. In *this* story, our heroine is Mikoto, who is one of only 7 students whose power has been graded at 'Level 5'. Her power apparently involves zapping folks with electricity, and her specialty is using an arcade token as a magical bullet and firing it at people at several times the speed of sound. She's pretty awesome, and the first volume involves seeing her trying to go about her everyday life while dealing with mad bombers, groping best friends, and the one boy who's unaffected by her powers... that hero I mentioned earlier.
Honestly, there is the start of a serious plot introduced here, with the so-called "Level Upper" that will allow you to increase your power level with little to no effort. Given this is a Japanese manga, I'm sure we know how well THAT will go. But for the most part, we get goofy scenes of Kuroko, Mikoto's best friend and a teleporter, trying to hit on her and being completely unsuccessful in a dumb "oh hey, it's the comedy yuri girl" way. Misaki herself seems to have a crush on Touma, the hero of A Certain Magical Index, who shows up here and gets a "reintroduction" scene since he's apparently going to be appearing both here and in the parent series. And there's one or two other characters who clearly will get fleshed out over time.
It's good simple fun, but it's not gripping or exciting. Aside from the bomb threats, not a lot happens here. If you're a fan of Index and its universe, you'll really enjoy it. If you have no idea what this is, but like the idea of a modern-day Lum firing electricity at bad guys, you might give this a shot. Everyone else can probably safely skip it.