You're not going to see gunpowder residue without firing quite a few shots. The test is far more conclusion than a visual inspection.

Well, he was shot over a dozen times at close range (probably while sitting with hands up), and both palms had exit wounds.

I’m not sure how well visual inspection would work with all the blood and other carnage.

However, I’m also not sure if trace gunshot residue will show up on current forensics tests if multiple people empty their clips into you from a few feet away.

There’s also the question of why they shot him over a dozen times while he was unarmed.

While true, how it was explained to me was that the autopsy did a visual inspection, while the GBI test, which was done afterwards, was a more thorough chemical one. However, I don’t have a source on this one.