[I just bought an old Iver Johnson 16 gauge single-shot shotgun and have no idea what the chamber length is.]
[ I would like to know if it is a 2 3/4 inch chamber just in case I needed to use one.]
Obtain a wood dowel that's will barely slide into the chamber.
Wrap the end with masking or electrician's tape (even with said end) until it's a snug sliding fit in the chamber at that end.
Slide the wrapped dowel into the chamber being measured, slowly & carefully, until it's forward motion is stopped by the chamber's forcing cone.
Do not force it as deep as you can, or you will distort the tape/shim, and thereby alter the measurement.
Mark the rear of the dowel at the rear of the barrel/chamber.
Withdraw the wrapped dowel, and measure the distance from the tip of the wrapped end to the mark - you should have the chamber length.