I had never read through this thread before and a lot of posting has occurred on related subjects in the last 11 years, but I though I would add a quick note.
I own a steel barreled back action gun from around 1869. It has a 12 gauge barrel and chambers but the only info available for the research letter was that it was sent back to the factory in around 1901 to be rechambered for "modern" paper shells. It had originally chambered for the 12B cartridge. The bore diameter of those guns was nearly the same as the shell case diameter. This presented a essentially straight bore with no forcing cone.
I suspect a lot of the first model guns were for the brass 12 gauge shell. A lot of these could be mistaken for 13 gauge.
I will also note that of all the 12 and 10 lifters I own, this is the only one with a true bore dimension. All the other 12's and 10's are either 11 or 9 gauge. These are up through the early 80's guns when, according to TPS, the switch was made to true bore gauge barrels unless ordered otherwise.
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