Most humane way of execution -- by gun?

From the New Yorker's Henrik Hertzberg:

It’s quick, it’s inexpensive, and it’s not as messy as you might assume. No one knows for sure how it feels, obviously, but I expect that because the bullet instantly destroys the brain stem, the pain, if there is any, is minimal. You’d feel the blow, I imagine, like getting slapped on the back really hard or tackled unexpectedly from behind, but it’d but just wham, lights out. No digging around for veins, no lingering agony.

When telling a friend about this, he suggested just loading people up on morphine until they die.