These photos show my 12 gauge chamber gauge. The bottom line is at 2 9/16 exactly. The next photo shows the gauge inserted into the chamber. It is indeed 2 9/16. The RST shells are about 2 1/2 inches exactly. They are fine for these guns. I really don't like to see 2 3/4 inch shells used in them.
Chamber Gauge by
jakefromclemson, on Flickr
Chamber on Trojan by
jakefromclemson, on Flickr