Unfortunate the site doesn't have a graph of loudness vs time.
They used real cannons, and the recording & mastering engineers did their best to reproduce them faithfully. It is one of very few albums I have with a prominent warning sticker on the album which is there for a good reason, not for bragging rights - it basically says that unless you are quite conservative with the volume knob, your speakers will die once the cannons do their thing.
Your comment has prompted me to do something I've been thinking of doing since I got that album - open it in Aucacity and see what the DR actually is - during the 1812 crescendo, it peaks at -0.02dB[FS]; RMS is -24dB.