Think of the chamber and the length of the bore up to the stuck bullet as being the whole chamber in this case. Lots of room for pressure build up and gas expansion. Pressure build up isn't any more extreme than Larry's example of a breach seated bullet (or artillery projectile in a cannon) and given the expansion area or volume, probably much less. Try a cast bullet level load of Unique or similar powder and you'll find the bullet gone. I would lightly lube the bore in front of the bullet to ease its travel, but that really is optional.it's not really that much different that shooting a breach seated bullet.
And as most posters insinuate, don't try and shoot it out with another bullet. That's an obstruction and the cause of many a bulged and ruined barrel.