My firemaking level is 79, which I would consider to be somewhere between a low level and a high level. How often can I expect hand cannons to explode on me?

I'm not quite sure but I know I've only had roughly 4 of them explode on me in a few thousand shots, although my FM level was higher than yours when I used them.

I'm not quite sure but I know I've only had roughly 4 of them explode on me in a few thousand shots, although my FM level was higher than yours when I used them.

From 1 shot up to a few k of shots is the range it can explode. There aren't accurate data to tell how much firemaking is affected but I expect you should average at least 250 shots more each cannon at 99 than you can at 61.