This is an engineering drawing from 1907, but from the beginning Hunter Arms chambered 12g guns 2 3/4", and 10g 2 7/8", and indicated on the flats if otherwise

[Linked Image from photos.smugmug.com]

The forcing cones were cut 1/2" - 5/8". The 3 1/2" shell length is after firing, so with a push could fit in a 2 3/4" chamber.

Could someone please measure an unfired and fired 3 1/2" shell?