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

http://en.wikipedia.org/wiki/Cannon#Materials,_parts,_and_terms

From that pic, you can probably find the name. Most likely it is the muzzle moulding, which doesn’t really have a purpose aside from being the part right before where the muzzle is cut flat.