You're mixing up two different issues. If your Fox is from the Philadelphia era, it was made with 2 5/8" chambers because that was a common chamber length for American 12ga guns at the time, and all kinds of 2 5/8" shells were available. If it's from the later Savage era and is marked 2 3/4" but has 2 5/8" chambers, then it's because Savage intentionally short-chambered the gun to get better patterns from the old paper case shells.
However, there is no advantage in shooting longer PLASTIC hulls in shorter chambers. Modern plastic wads take care of the shot protection/pattern improvement issue. So what you're doing if you're shooting modern American 2 3/4" shells in a 2 5/8" gun from the Philly era is that you're using shells that may generate higher pressure than the service loads for which the gun was designed. (Service pressures increased about 1,000 psi from the old 2 5/8" shells to the new 2 3/4" shells.) And on top of the pressure increase from the shell itself, you've got another pressure increase from the longer shell in a shorter chamber. Combined, that might be as much as 2,000 psi. The gun will probably handle it OK, because as you noted it's pretty stout. But in the long run, you may not be doing it any favors.
Reloading modern 2 3/4" hulls for old guns with short chambers is a different story. Because you control the pressure when you reload, you can work up loads which are well below the maximum service pressure for which an old 2 5/8" gun was built, which was about 10,500 psi. Not hard at all to produce even 1 1/8 oz hunting loads that only run about 7,000 psi. So even if there's a pressure increase of 1,000 psi from the longer hull in the shorter chamber, you still have a comfortable margin for error.
Long answer, but it's not exactly a straightforward issue, especially when you throw in the practice of intentionally short-chambering guns back in the paper case era.
Last edited by L. Brown; 01/13/14 08:53 AM.
