If one is planning on regularly shooting an American vintage SxS, and it has/had "short chambers," I see no harm in opening it to a standard 2-3/4" chamber AND adding longer forcing cones at the same time (assuming there is enough barrel thickness). I have had my gunsmith do so to several of my SxS shotguns.
Pressure is lowered at least marginally, and the useful life of the shotgun is probably extended. It does no harm...
gold40
Jerry, I think that advice is good maybe 95% of the time. Maybe even more. But I don't think it would be a good idea, for example, to take an Ithaca Flues 20ga, especially if it's one of the lighter ones, and punch the chambers to 2 3/4" and lengthen the cones. At the least, I'd certainly want to take some wall thickness measurements where I intended to have metal removed if I were to contemplate such a modification. You're certainly not making the gun any stronger by removing metal. And while MOST vintage American guns are pretty stout, some aren't. We've already heard of Flues frame failures, usually light 20's. The problem, at least in a couple cases I recall, appears to have been "light" promo ammo (which is not really light in terms of pressure, nor velocity), 2 3/4", in short chambers. If enough metal were removed to extend the chambers to 2 3/4" and lengthen the cones, I think one might have a legitimate concern whether even ammo at pressures appropriate to the pre-modified gun might not cause problems.
