I use a bore micrometer (internal micrometer).
I used to have one made by Custom Shooting Products of Omaha.
Do they still exist as a company?
The brass stepped guages on a keyring are about as much use as a chocolate fireguard IMO.
Unless you know your bore size they are only a rough guide.
Totally useless if they are made with 0.729" as the standard and you are measuring choke on a backbored barrel.