Here's the real problem with American guns: Back when the changeover was made from short chambers to 2 3/4" as the more or less standard for 12-16-20ga guns, it was not really a problem. Looking at my 1940 Shooter's Bible, I find WAY more factory shells available in the short lengths (2 5/8" for 12ga, 2 9/16" for 16ga, 2 1/2" for 20ga) than I do 2 3/4" shells. But after WWII, by which time American gunmakers were no longer producing short-chambered guns, those shells began to disappear. The common solution when you consulted your local gunsmith: just go ahead and lengthen the chambers.
The problem with that simple solution was that most gunmakers proofed the short-chambered guns to a lower pressure standard than those with 2 3/4" chambers. The difference wasn't huge--around 1,000 psi or so. However, by the time you figure that you're going to get some additional pressure (although again, not a lot) by firing a slightly longer shell in a shorter chamber, you may now be pushing 2,000 psi or so over the service pressure for which the old American short chambered guns were designed. Because most American guns were somewhat overbuilt (at least in comparison to British game guns), this usually was not a problem as far as "catastrophic failures" go. But it may well result in a shortened lifespan for the gun in question, in terms of shooting loose, if fed a regular diet of factory 2 3/4" shells.
That's why, as suggested, it's very wise to check the barrels on any American gun on which the chambers have been altered. (A good guideline: if it wasn't originally stamped 2 3/4", it probably started life with short chambers). And to stick with either low pressure factory loads (like from RST or Brit/Euro ammo makers) or low pressure reloads in guns that have, or originally had, short chambers.
