I cannot provide any documented guarantee, however it is estimated it takes approximately 10-15ms for an external stimuli to be processed by the brain. Since a 12 gauge blowie is extremely likely to result in a complete explosion of the head and brain I would say the likelihood of the brain registering any pain sensation is extremely low. Even if somehow it miraculously didn't result in total brain destruction, the concussive force of the blast should render one instantly unconscious without any time to register the impact of the blast.
Assuming neither the weapon or ammunition fails, I would consider the 12 gauge blowie probably the most reliable method from a speed, lethality, and pain free perspective. There is however, the matter of the awful mess and the fact I'm too much of a pussy to go that route.