20 gauge chamber length

[Drew Hause]

What Dave said.
A.P. Curtis published an article in the March 1938 American Rifleman entitled “Advantages of Short Shotgun Chambers” (courtesy of Larry Brown):
SAAMI, assembled in serious conference on March 26, 1937, passed the following resolution: “That an appropriate warning label be placed on all boxes containing smokeless powder shells, cautioning the consumer against using them in short chambered guns and also in guns with Damascus barrels and guns not in first-class condition.” The motion was made and seconded by representatives of two powder companies.
That same conference also passed a resolution requiring: “That all guns be marked so that the consumer will be able to tell the chamber length, as for example by marking 2 3/4 inch chamber etc.”

Hunter Arms lengthened the 20 gauge chambers from 2 1/2" to 2 3/4" in 1936.