There are quite a few 16 ga American guns around with short chambers as well. You might find that with the older guns with short chambers (and short forcing cones) that the 2.5" shells pattern better.
I picked up a few cases of Gamebore shells and couldn't be happier. Probably run around $11 a box, so really not much more expensive than factory 2.75. With the fiber wads they pattern better than the plastic ones in the guns that I own. I have found them on the shelves at Cabell'as as well so not too uncommon.
Standard 1oz at 1150fps for pheasant and other upland has worked well
As far as when it is possible to lengthen chambers. I believe that there should be around .090 of metal left around the end of the chamber in order for it to remain "in proof" as far as most of the Europeans are concerned. Some of those German guns can taper pretty quickly, especially on drillings and the like.
If it looks like a chamber has been lengthened, I won't buy a gun before I do the math to see if it should be safe to shoot. I have seen some where the contour was about the same thickness as the rest of the barrel. Too close to the chamber, too thin for me.
Cheers
Tom
