I Agree with all the Old Colonel said on this except for one thing which I will take exception to. That is that most guns which were intentionally made for use with all brass shells were bored oversize not undersized. At the time these guns were made all shells were using card & felt or fiber wadding. The brass shells have thinner walls requiring larger wads, thus the oversize bores. Commonly a 12ga brass shell was loaded with 11 gauge wads, a 16 with 14ga & a 20 with 18ga etc. Note also that ion the very early days wads were essentially bore size so in those days brass shells used 2 sizes over as 10 ga wads in a 12 while a paper shell used 1 size over as an 11.
At some point the makers began making the wads larger than actual bore size so for a 12 ga paper case you used 12 ga wads, for the brass 11 etc.
Per 2-Piper, you are correct I misspoke, traditionally if different sized the bores would be undersized not oversized
If the bore proof mark is 15mm (0.591 inches) That number would make sense as the original choke marking for a 20 gauge which would be .025 from a classic 20ga bore of .615 and makes sense.
The fact the gun near chambers the 20ga snap but not quite, when my 24 won't admit into the chamber any part of a 20ga shell or snap cap. Makes me think it can't be a 24ga.
Really need pictures of all marks and barrel stamps
