Where did Hellboy get the hand gun Hellboy calls the "Samaritan"?

I do believe you are looking at a Hollywood-romanticized version of the Gilon revolver, which was chambered in .577 Boxer (also called .577 Webley) indeed the cartridges were so large the cylinder was chambered for 4 rounds. The real bullets were nowhere near as long and pointy, being regular LRN (Lead Round Nose) typical of the latter 1800's, as all calibers were.

The short and squat cartridges you see throwing large pieces of lead were made that way because that's the best use of black powder, which was quite low pressure. if Hellboy had modern technology and smokeless powder (nitrocellulose) he probably would have preferred something in .500 S&W which would have been a lot more effective for his purposes.

There were other revolvers chambered for that cartridge but the Samaritan most closely resembles the Gilon. Yes, there were actual guns like this but not beefed up weighing 10 pounds.

this guy has one although he says it's actually .600 caliber. (it's not) here you go... huge bullets, 4 round cylinder and all.