The 12-gauge Nitro Specials were chambered for 2 3/4 inch shells from the get go. The 16-gauges were chambered for 2 9/16 inch shells from the introduction until 1934 when they went to chambering for 2 3/4 inch shells. I think the 20-gauges were all chambered for 2 3/4 inch shells, but I have never found confirming company literature. The .410-bores were chambered for 2 1/2 inch shells until 1934 when they went to 3-inch.