I would interpret it that Jourdonais would have to Draw! twice looking for heart to avoid a shot from Slab. Jourdonais effectively always has a barrel built in, but he still needs to dodge Slab's ability.
skelebone (#38861), Almost, but not exactly. Since Jourdonnais has a Barrel built in, he may "draw!" once to see if he plays an automatic Missed!. After that, if he drew a Heart he will need a real Missed! from his hand to dodge Slab's shot. Otherwise, he will need two Missed! cards, as anyone else.