Young Tom Scott has recreated the rock cannon from Betws-y-Coed:

https://www.youtube.com/watch?v=J3F2odr2MsQ

worth a watch!

I wish I'd known about those when I was younger, though my parents are probably very, very grateful that I didn't (especially as we lived next door to the police station).

The book has eluded me for 15 years. In various wanderings I've come across about 100 cannon rocks but there doesn't seem to be any significant reference to them excpet GR Jones's book

I am fairly sure at least one of the cannons was used in recent times and filmed...maybe that's on goontube as well.

Certainly at least one more on goontube:

https://www.youtube.com/watch?v=XOG2-J0-R8g

The information board near the cannon suggests that it was fired to celebrate the investiture of Charles in 1969, but it definitely wasn't. I would have remembered  the massive fuss  -  elf & safety, fire precautions, crowd control etc. (and I would have heard it) . It was probably created and used for the coronation and the investiture in 1911, but that was too long ago, even for me to remember.

Some of the cannon in the book are difficult to find and maybe not at the grid references given.