I actually disagree with that. We're in morbidreality after all, so I'll explain why.

A concept mentioned in both English law and the US constitution is that of avoiding "cruel and unusual" punishments. Death by shooting is far more common than death by hanging, gas, electrocution, or lethal injection. According to the Wikipedia statistics, there were 10.3 deaths per 100,000 people related to firearms in the US in 2011 - that's around 32,000 deaths.

In terms of cruelty: personally, I would determine cruelty by the real or percieved awfulness of the death, as well as the anticipation of imminent death. The thought of electricity frying my brain seems horriffic to me, I don't want the last thought on my mind to be waiting to drop through a hole (and potentially have my head ripped off or strangle to death), gas has too many holocaust connotations to even consider; and lethal injection, while being (disputably) painless, takes a good deal of time both in preparation and once the process has actually started, the entirity of which I'd be all to conscious that my life is gradually ending.

So for these reasons, if I was facing execution and was given the choice of a punishment I think I would choose firing squad. It just consider it to be the most common, and relatively quick and painless.

And in the overall scheme of capitol punishments you're likely to find in third world warzones, you're forgetting "methods" like stoning, beheading, vivisepulchure, or even just a bit of old fashioned "throwing guys off the roof of a building because they're homosexual". Compared to those, execution by a professional firing squad is basically a punishment fit for a king.