Jack usually takes punches by not being hit by them. But some things just blind him, and he doesn't see this coming.

Dean's punch smacks straight into Jack's jaw at the same time as the first shot he's taken on this world goes harmlessly through a wall. The pirate almost goes down, certainly staggering.

Then his eyes snap up to Dean, and slide unspeakingly to Jo.

But he doesn't bring the gun back up.