If indeed the chamber ends with square or otherwise sharp shoulder, the issue goes beyond merely matching the length of the shell to the chamber. The thickness of the mouth of the case needs to be considered. A new or once-fired paper case is about 0.025" thick at the mouth and permits a nice, undramatic transition to the bore. A plastic case (not v compatible with BP) or a brass case will measure about 5-10 mils thick at the mouth and will offer the payload a much more intimidating look at the forcing core or, as we discuss here, the abrupt end of the chamber. After accurately ascertaining the chamber length, the type of shotshell case should be chosen so as to minimize abrupt transition into the bore itself.

In the case of the step at the end of the chamber there really isn't a problem as plastic and/or paper shells are thicker than the metallic cartridged these guns were designed to shoot. The step measurement is approximately the same measurement as the thickness of the metallic shell wall.





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Easy to tell the measurement of the transition by running a bore gauge through the chamber into the bore. Unfortunately, a Skeets micrometer will not measure chambers in ten or twelve. I use Galazan's gauge that is up around $500 now.