Removed the forearm. Proof marks did contain an open oval with 12-70 on both barrels were the only numbers present. The 12 being gauge and the 70 maybe 2 length chamber. Probably not an original 3", but mention was made that Browning had a US service center that would open the chambers on request. As mentioned to Bob and Researcher, the forcing cones seem to rougher than the balance of the chamber as in they were not polished when the chambers were cut. Forcing cones appear to be elongated and not abrupt as I would think they would be.
