You should mention that the guns were not loaded with anything more than gunpowder and wadding. It does not make it less barbaric but people think that cannonballs or grapeshot was used.
It might have been true that other countries took their time in executing men and tying them to the cannon for days. However the British executed quite quickly. They probably only executed around maybe 15 by this method 12 on one day and that was probably because the men were fanatics.
Tir is a first-hand account by witnesses, that told of all being given the chance to turn Queen's evidence and a few did take that opportunity and were pardoned a few more who were condemned to death were hung, which leads me to think that those who did suffer that kind of death were totally unrepentant to the atrocities that they did to the British woman and children during the Indian Mutiny. They went to their deaths according to eye witness again calmly and then began shouting insults and saying they would do it all again. All of them showed no remorse whatsoever to what they personally did to white women and children including babies. time and time again.