What was the purpose of the circular ring on “old” cannons right before the muzzle

Question

I’m guessing it’s for structural reinforcement, but not all cannons seem to have it, and I’m having difficulty understanding how making a small ring which seems to be less than a cm thick extra would make enough of a difference to warrant adding it. Also, does it have a name?

EDIT: Image: https://imgur.com/5oGdoAm

In: Engineering

Answer 1

[This 1832 reference](https://books.google.com/books?id=xlJXAAAAYAAJ&q=swell+muzzle) has “the diameter of the swell at the muzzle is so proportioned in all to the respective base rings as to give a dispart of a degree.” This strongly suggests that sighting was at least part of the reason.