Internal hinges are a common style of hinge for many home safes, and gun safes. This type of hinge attaches the door of a safe to the body by the means of a swinging hinge within the safe. Internal hinges cannot be seen from outside of the safe.
Some great reasons for an internal hinge is for looks, the inability to cut off the hinge in an attack, as well as to keep the outer dimension of a safe smaller for access to rooms with doors that are under the standard width.
Drawbacks of this hinge style is with fire rating in some models, only a 90 degree door opening, and the inability to pull the door off of the safe for moving a safe within a home.
Popular manufacturers that use these hinges are Liberty Safes, Fort Knox, Champion Safes and many others.
To see more a comparison between this type of hinge and the external hinge type (as well as the 2s Crane Hinge), check out my video about this here.