hrm, RightStuf can be unreliable… poking through kittymedia’s product search turns up
“Heartwork - love guns”
Yuu Asakura was a mild mannered university student until he found a mysterious duffle bag with a gun and orders for an assassination. He was terrified at first, but when he held the smooth metal of the pistol in his hand, he started to become someone else. Capable of anything, Yuu takes the assassination job and enters a world of nonstop sex, blood and guns. When he returns to school with his newfound abilities, he finds it no trouble at all to get any girl he wants. The syndicate and the police want a piece of the new guy, but the one Yuu has to worry about is the gun’s original owner.
But this seems to be a DVD. Adding ‘???’ to searches comes up withthis which does reference Bible Black and Discipline although I haven’t translated it yet. Looks pretty old if that’s the right game.