Sherman Bell did the research for us, finding that there is no appreciable rise in pressure when shooting 2 3/4" shells in 2 1/2" chambers. The advice from Bell and others is to shoot shells of a load that was made for the gun, regardless of the length of the hull. I have shot 2 1/2 DE, 1 ounce factory 2 3/4" 16 gauge loads in short chamber 16s for decades with no ill effects on the guns. Drilling out a short chamber gun is a waste of time and money and possibly to the detriment of the value of the gun. The Brits do so much to destroy their gun barrels by excessive honing of the bores, and I won't add to the damage by drilling out the chambers.
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