How would one go about estimating the break DC of a creature's stomach?

[chronomatophobe]

You know, if a character were to be swallowed whole then use the expansion power (or something similar) to become too large to fit in a creature's stomach?

[BowStreetRunner]

As long as the creature is alive, you cannot break its component parts. Once you have killed it, the corpse becomes an object. Leather or Hide has a hardness of 2 and 5 hp/inch of thickness.

[Shining Wrath]

What is the creature made of? Probably something like leather, so use that.

But the leather is reinforced with ribs and muscles, so add accordingly.

Also, the weakest points in the walls are likely to be the esophagus and duodenum, so your character may legitimately get extruded up toward the mouth and down into the intestine. That's probably not what they had in mind and might be fatal.

[Grod_The_Giant]

I'd go with a DC equal to the amount of damage you need to deal to cut your way out. Or some variation thereof.

[Myou]

Don't forget to allow a +2 circumstance bonus for masterwork wafer thin mints.