I was following a series but I can no longer find it, and here's what I remember, though some of this might be inaccurate.
It was drawn by an artist that drew mostly yaoi, but the series itself has some romance but nothing explicit.
The story was about a high school boy who woke up and found his arm could turn into a gun and that the soul of the gun can control his body...? (I think the boy might be the reincarnation of the gun)
The gun used to be a "nun" whose alchemist friend (the guy who designed his gun weapon) was burned at the stake during the a religious inquisition...?
The art style reminded me a bit of KOTOBUKI Tarako who did Sex Pistols (but it's not by her)

More details:
The boy is being taken care of by his cousin who looks exactly like the nun's alchemist, but it was implied that the real reincarnation of the alchemist might be someone else, a classmate.

Does this ring a bell?
I remember the volume 1 cover had the main character standing sideways and looking back or at the viewer.