Wayne brings up the reason for knowing that the chamber is: ie: wad diameter.

A thin brass case needs larger wads due to the thinner brass.

Turned brass cases will likely be thicker, however, if the chamber & bore is cut for thin factory brass, your wads will need to be larger in diameter.

Paper cases used smaller diameter wads due to the thickness of the paper case.

If you use thick case walls in a chamber that is meant for thin brass, the wads will likely be undersized for the 'larger' bore.

It is a good thing, to know, rather than guess at what you have for a gun.