All of the early 20 ga .model 1912's had 25" barrels, and they all had 2 1/2" chambers. I have one of the early ones, and I lengthened the chamber to 2 3/4" with no other changes and it works fine. I don't know the original stock length, but it seems a bit shorter than later guns. mine is the only one that I have ever managed to kill 5 wild quail on the covey rise with.