Thanks for all the replies.I have decided to go the 3 1/4" route as explained above. The next question that perhaps Gunman can help me with is regarding the pressures relating to black powder re-proof for both 12 and 10 gauge and again whether chamber length plays a part??? Sorry for belaboring the issue but would be interesting to know if pressures are similar or not.
BTW Gunman, I am also disappointing that the proof house is unable to recognize the issue of standard proof for vintage guns...


Better to keep your mouth shut and be thought stupid,than open it and confirm.