Found this in the BBC News: Guy Fawkes plot 'was devastating'. I never knew the man had been in the army before.
Guy Fawkes could have changed the face of London if his 1605 plot had not been foiled, explosion experts have said. His 2,500 kg of gunpowder could have caused chaos and devastation over a 490-metre radius, they have calculated. Fawkes' planned blast was powerful enough to destroy Westminster Hall and the Abbey, with streets as far as Whitehall suffering damage, they say.
Early in the morning of 5 November 1605, Guy Fawkes was discovered in a cellar under the House of Lords with 36 barrels of gunpowder and a 'slowmatch' to ignite the explosive. The plan never came to fruition, and Fawkes, like the annual population of straw-stuffed effigies, faced a painful execution.
According to explosives expert Dr Sidney Alford, Catholic Fawkes used substantially more gunpowder than he needed to destroy Parliament. In a report published in the New Civil Engineer, Dr Alford calculated that Fawkes and his fellow conspirators went for an overkill, filling the cellar beneath the House of Lords with 25 times the explosive necessary to bring the building down. Guy Fawkes was no amateur in explosives. Before he became a professional plotter, he worked in the army, where his job was to pack gunpowder. Therefore if he used 25 times too much gunpowder, maybe it was no accident. David Reid, spokesman for the Institute of Physics, said: "This throws into question exactly how much damage Guy Fawkes intended to cause."
I love news about Guy Fawkes Night. The University of Wales people assumed the explosion took place above ground and that gunpowder was equivalent to TNT for their calculations, so the damage estimate is a rough guide. I posted about the history of Guy Fawkes last year.