3 to 1 rule as some call it is a minimum standard guideline for safety purposes for cannons. Yes it is is possible to build a barrel with thinner walls, safely. Your cannon is a prime example why it you shouldn't.
The barrel doesn't convey confidence by its appearance. Walls are too thin, trunnions have suspect attachment, it doesn't look like a cannon, it's on a flimsy weak looking carriage. Nothing about its appearance conveys confidence that the maker had any idea about how to safely make a cannon.
You aren't going to always be around to explain--the cannon needs to "speak" for its self. I don't think your cannon is unsafe as far as barrel bursting. I do have some concerns about trunnion attachment.
I am a long way from my college metallurgy class and don't recall the various ranges, so I will let someone else comment on the effects heat treating to Rockwell scale c 33/35-- brittle? I don't really think so, but someone with more knowledgeable can comment
Overall you cannon just doesn't look safe.