I think cruel and unusual is one of the issues. Something may be cheap efficient and painless but it's not historically used it could be argued it's cruel and unusual. Unlike hanging, firing squad or electric chair which all sound cruel but are historically not unusual. Hence why they are still used when much more painless and quick methods exist.
The Definiition of Cruel and Unusual has 'never' included the firing squad, which was, for generations, considered the honorable method of execution among the military.
Cruel and Unusual was drawing and quartering (yanking apart by horses)... or things like dismeboweling, lighting the bowels on fire then cutting into pieces and having your head displayed on a pike... etc. Sick shit. The British loved it.
Firing squad was always considered an honorable way to be killed for crimes or failures (in a miliitary sense.) Much the same as Seppuku was considered an honorable way out for the Samurai class... though we would think of it as pretty gruesome.
The Firing Squad was brought back by Gary Gilmore in Utah in the 1980's. He asked for it.... he got it. He was a shitbag. So he set the prededent... not 'the citizens.'
Fuck them all. If it were up to me, I'd use the Bellowing Bull on all of them.
Sirhr