Parker Bros. "standard" chambering for their 20-gauge guns was 2 3/8 inch intended for the 2 1/2 inch shell. They would on request chamber for the 2 3/4, 2 7/8 or 3-inch shells, but special requests on a gun built for stock like the Trojan would be very unusual. Even though the one ounce progressive burning powder 20-gauge load (Super-X) was introduced in 1922, put up in Western Cartridge Co.'s 2 3/4 inch FIELD shell --
Parker Bros. continued to chamber their 20-gauge guns for the 2 1/2 inch shell into the 1930s.