So, I was watching this episode of Critical Roll in which Scanlan and Vax dimension door into the stomach or an ancient black dragon. Normally, and ancient black dragon has an AC of 22 due to exterior natural armor. However, inside a stomach, that armor is not present. It is soft tissue and unable to use any sort of dexterity to outmaneuver attacks. Matt Mercer, the DM, doesn't really adjust this when they are play, stating that they are squeezed in the stomach and cannot hit for damage effectively. I find this to be absurd. The stomach and internals would have an almost guarantee to hit, and in the case of Vax the rogue, sneak attack, but this was dismissed. What would be a fair call on this. I find striking the internals of a gargantuan creature would lower the AC considerably and with another PC within 5,' the sneak attack would be arguably applicable. Any thoughts from fellow DMs would be welcomed. Thanks