HsPS-15 (also called 'Large Weapon' (ラージウェポン Rāji Uepon?)) is a technology developed by Academy City. It is the newest version of Powered Suits as of Volume 14. It first made it's appearance in Railgun Anime Episode 12 cleaning up the the debris after the fight between Mikoto and the AIM Burst, and in the Vol.14 of the novels. It can be worn in addition to Anti-Skill's riot armor , but AI can also be programmed into the armour to make it work without a pilot .
Haimura had the most trouble in the design for the HsPS-15. After realizing that the suits were worn like a "suit," rather than a piloted, and had trouble with the anatomy of a man wearing robotic suit. After several drafts, Haimura finally settled on a design, getting the idea from Overman (King Gainer), and making it like manmade muscles, chanmail, and plate armor, worn over a human framework.
And with little time to spare, Haimura managed to finish the design for their weapons.