Posted by Drew Hause on September 12, 2015, 8:09 am, in reply to "Re: Chamber Mark"
The remaining Hunter Arms records available for a Research Letter are unlikely to contain specific instructions for a special order chamber length.
If your smith is using a simple rule gauge to measure the chamber length, I would suggest a second opinion Bob.
I have a 1906 00 which had been subjected to chamber lengthening to 3 1/4", and forcing cone lengthening. Fortunately, most 00 breech barrel sections are stout, and the wall thickness was still .118" right and .111" left; adequate by any measure.