Forgive me for the lack of specific detail but I do not have my books with me right now but here are a few observations on proof sizes:
Original dimensions of nominal proof sizes were not all 10 thou apart. They were the theoretical size of a ball of pure lead etc etc.
The proof size marked is a nominal one and unless by design of the barrel borer, has never been intended as the exact bore size, just an indication of what nominal proof range the bore is in.
When proof sizes went to imperial measurements, 10 thou was the figure commonly found around the 12's and 16's but I think you will find it drifts for smaller bores.
New guns were normally bored as close as possible to their nominal size, eg 0.729", but there was no obligation to do so.
For reproof, wall thickness would not sacrificed to the God of bore size and I have seen guns unfortunately reproofed only a couple of thou inside the next proof size up. This was particularly galling in the old imperial days, less so with the new subdivided metric sizes.
The proof houses have often marked reproofed guns as the next proof size up when their actual 9" measurement is very close but not quite there. I have seen guns as much as 3 thou tighter than the proof size they have been marked with.
When the metric measurements were introduced there was a transitional period when one could have the proof marked in imperial or metric. At this point the 0.729" changed to 0.728" as this is closer to 18.5mm. Also the 0.2mm gaps could be represented by 4 thou nominal proof mark steps, eg. 0.732", 0.736" etc.
When the metric proof stamps were introduced, this finally broke the connection with the old bore definition and the metric procession proceeds in 0.2mm steps throughout the bore range.
Just a new observations. Enjoy!
