Loiner
Senior Airman
If done in the correct manner the Firing Squad is one of the most painless executions available as bullets travel extremely fast, meaning the person would more than likely be dead before they knew what was happening.
Much less painful the hanging or getting fried.
Looking at the technicalities, the shot is to the heart, if done correctly unconsciousness should be within seconds, but I guess the worry is if the heart is just missed, that could leave the recipient in major agony for quite some time. I've read that the electric chair produces unconsciousness in a fraction of a second, and lethal injection puts you to sleep before the fatal dose is administered but both have gone badly wrong before.