The gun was given to Hellboy at a young age by the Torch of Liberty, it was named after the samaritan in the biblical story "The Good Samaritan".
The gun was built specially to fight evil and supernatural enemies. Its metal is formed from various church and biblical components including: silver church bells, nails from the crucifixion of Jesus, various blessed chalices, and many more forms of silver and copper (known elements for fighting evil). It's handle is actually carved from the cross Jesus was nailed to.
The gun is large and unwieldy, it can only hold four rounds and Hellboy can only load and fire it with his normal left hand. It is highly resistant to attacks and shoots rounds with incredible force.
Hellboy has used various special bullets for different reasons. He's repeatedly used tracker bullets that produce a green florescent material (as seen in the film) and bullets with certain materials such as garlic, silver shavings, bones ect. in the tip that are known for their supernatural effectiveness.