Read somewhere about a safe that had some sort of a seal that expanded when temperature reached a certain level to keep heat out. Looked on their website and didn't see a reference to that feature but may be the key to the 2 hour rating. I did note that they mentioned that their 2 hour safes were OK for document storage as well as firearms.
Many gun safe manufacturers use drywall as insulation and the thicker the drywall, the higher the rating, I'm sure. The main reason gun safes are not considered fireproof for documents is that they cannot be made air tight (as regular safes are) in order to prevent condensation (and resulting rust) on the firearms inside.
W/proper insulation and a seal on the door, the rating cqn be increased. You might ask them how they achieve their higher rating.
Regards,
hm