Does anyone know this manga? Or possibly manhwa? I remember reading it back in the mid 2000s. It has something to do with immortality and an undying witch.

The main character is a young man who is attacked along with his older mentor on a train in the beginning of the story, possibly over a secret they have discovered to immortality. The young man uses a lever-action rifle or shotgun to fight back and is his weapon for the rest of the story. He eventually teams up with the witch and possibly a Frankenstein-like girl. I also remember the old man having a daughter or relative who teams up with a detective in a subplot.

Please help! It's been bugging me for a while.

https://www.mangaupdates.com/series.html?id=34325

Figured it out myself by searching the forum. So it was licensed....