A very easy way to check chamber length on any shotgun is to use a index card, roll it lengthwise and insert it in the chamber and let it conform to the chamber size. Then gently slide it until it stops, mark it and try it again. Works every time and is simple to do anyplace.
Jon, years ago I only shot 20 gauge and started reloading for skeet. I used only Winchester AA compression formed 2 3/4" hulls. They all seemed to be consistent in length. I shot them out of a Winchester 101. Since then I have not reloaded any 20's but still have components to do so, but again they are 2 3/4". Since I started shooting and collecting doubles, mostly L.C. Smiths, the chambers for 20 gauge until sometime in the middle 1930's was 2 1/2", after that they went to 2 3/4". So I cannot say anything about loading short 2 1/2" 20 gauge, however I do reload 2 1/2" 16 gauge. The chambers for 16 gauge L.C. Smith's until late 1939-40 were 2 9/16" chambers. I use a Mec 600 Jr. and use a 1/4" spacer under the 3-5 stations to raise the hull for powder/shot, start crimp and final crimp. I use mostly Cheddite hulls, some Federals and a few black Remingtons cut to 2 1/2" .
From all the shooting you do you probably use a progressive machine like the Mec 9000. Not sure how short 20's are done in that.
