[Heff]

I have been reading through post with the search engine on various chamber lengths and gauges. Based on what I have been reading, the consensus seems to be that chambers were roughly 1/8" shorter than the desired shell to help seal pressures with the old fiber wads (2 5/8 for 2 3/4 and 2 7/8 for 3" in 12 ga)
Does this mean a 20 ga intended to shoot 2 1/2" shells would have 2 3/8" chambers? If so, can 2 3/4" shells be safely used in this configuration or should 2 1/2" be used?
I recently purchased a 20 ga made in 1911 that by my measurements with a machinist scale check out to be 2 3/8" chamber length to the front of the forcing cone.