Carl,
It is much easier to just use a micrometer to measure the dimensions of the bores. Remember, it is the barrel bore diameter (not the chambers) that determine the gauge of a shotgun. Shotshell diameters were very well established by the time the Parker Bros. shotguns (both back action and front action) were first being made. And the diameters have not changed since that time---except the 10B, 11B, and 12B have gone extinct.
You can use modern shells to perform a simple test of the chambers of an early under lifter Parker Hammer Gun:
1. A modern 14-ga shell fits perfectly in the chambers--gun was chambered for either a 14-ga. Paper or 12B brass shells. Gun could be a 14-ga but probably a 12-bore.
2. Modern 12-ga Shell fits--gun was chambered for 12-A brass or No. 12 paper shells. Gun probably a 12-ga but it could be an 11-ga Parker.
3. Modern 12-ga shell is loose and modern 10-ga shell will not go in chamber --- gun is most probably an 11-ga Parker.
4. Modern 10 ga shell fits---gun is probably a 10-gauge.
Richard
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