Might this be Nobunagun...?
It has a similar sounding plot, and the first chapter was released in 2011, so it's old enough to be the one you read. Also, there were only six volumes released in total.
To protect the world from the impending invasion of the alien species known only as "Evolutionary Invasion Objects" (EIO), the supranational organization DOGOO created special warriors through DNA manipulation called E-Gene holders. These E-Gene holders are the reincarnations of various historical figures. Using special weapons called AU balls, E-gene holders can call forth AU weapons with an affinity to their assigned historical figure and use it against the invaders. Sio Ogura is one of them, being the reincarnation of Oda Nobunaga. However, her memories of her past life are limited only by dreams about the military past. Going on a school trip in Taiwan, Sio gets caught in the middle of a battle between Evolutionary Invasion Objects and Dogoo's E-gene holders, alongside her classmate Kaoru Asao. When one of the E-Gene holders, the reincarnation of Jack the Ripper, gets incapacitated and Kaoru ends up in danger when confronting an Evolutionary Invasion Object, Sio uses Jack's AU ball to fully activate her E-Gene holder status. She gains her AU weapon, a massive machine gun, and assists the other E-Gene holders to put an end to the Taiwan invasion. After some reluctance, she joins DOGOO at Kaoru's behest.