When I visited Oslo we went to Akershus festning, where we heard this enormous blast that sounded like a bomb going off, it really startled us as we weren't expecting it. After recovering, we realized they were firing off some cannons there, I don't know if it's a regular thing or not. I was expecting some massive cannon based on the blast, but it really didn't seem all that large. I can't even imagine what it must have been like on the gun deck of a large warship with 30 or more cannons.