I have a Brit 16 gauge (Harkom) that has 2-1/2 chambers and is proofed for 3 tons. Have owned this gun for years and have experimented with 2-3/4" and 2-1/2" hulls and loads. Frankly, a low pressure load, regardless of whether it is loaded in a 2-1/2", 2-9/16", or 2-3/4" hull, is still a low pressure load. Loads for a 16 bore can produce pressures as low as 7000 psi.
In Fact, Stanbury and Carlisle in their book, Shotgun Marksmanship (page 35) stated, "...The length of a cartridge is not the same as the length of the chamber, but, with rolled turn over cartridges, the length of the FIRED case is the same as the length of the chamber. With some crimp closure cartridges the length of the fired case is actually longer than the gun chamber; for instance, a number of 12 bore cartridges intended for 2-1/2-inch chambers have cases which are 2-3/4-inches long. {...} the finished cartridge is only 2-1/16-inches long when crimp closed, as some of the tube is used in the closure..."
It is reasonable to assume from this that one may load a 2-1/2-inch LOAD into a 2-3/4-inch case and that this is, in fact, commonly done with factory 2-1/2-inch cartridges. I have done this for years for both my 16 bore guns, including the 2-1/2" Harkom. If you want to play it extra safe - cut the hulls off at 2-9/16" and use a roll crimper and overshot card.
Here's a recipe that I use for LP 16's in 2-3/4" WW16 CF hulls...
WW CF 1 oz. SR 7625 20.0 gr WW 209 Rem R-16 1155 fps 7,000 lup HSR
Any 16 gauge shooter, if he does not handload, is at the mercy of manufacturers and the market. Doubly true for those who own guns with short chambers.
