I answered on the basis of what most Parker 12's were intended to shoot when made. Guns found today could be badly pitted, bored out, rusted out, seams split, bulged or other reasons why they are not as made.
Most shooters and many gunsmiths do not own wall thickness gauges. Bore gauges are owned by a greater number of shooters than wall thickness gauges. A simple bore gauge will quickly determine if the gun is at or near nominal diameter for the gauge and as made by Parker. If the bore is far out of nominal ID, then wall thickness becomes necessary to know. Twelves usually have wall thickness to spare. The issue is more acute with 0 frame 16's or 00 frame 20's. Some 20's will be a factory wall thickness of .022". See Robin Lewis's excellent latest Parker Pages article.
In my opinion, and others may differ, it is not necessary to check the wall thickness of every gun, unless there is something about the bores that give a person cause for concern. I do however, check the bore ID on all guns I own or may consider obtaining.
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