The hellbreath is a gun that (as the title suggest) is a gun that shoots electricity and packs quite a punch in the damage department. If you are going for the Scrooge achievement I would be careful not to try get this gun Untill later on in the game as once you reach the church you can get it for free but if you are unlike me and didn't really play this game the first time the original was released and was curious and impaitent and had no idea.... Then you can do what I did which was purchase this gun on the "ARMORY" level for 250 (I belive) military grade ammo and farm the kills on the level "FRONTLINE".
The gun uses the pellet like ammo and has a gas lever which wil need to be pulled once it runs out which can really hurt you in the long run but this gun is a very good one to have when facing demons and librarians!
If anything needs changing please let me know in the comments!