A few things of note; assuming proofed after 1887 then 13 = .710"-719", 13/1 = .719"-.729+. 12 = .729"-.740" & 12/1 = .740"-.751". At this point no exact bore size was given, only that it would accept a plug of the size marked to a depth of 9" from the breech & would not accept the next larger size plug to that depth. Thus a .711" & .718" bore would have both been marked with a 13 with no distinction between the two.If proofed prior to 1887 then the whole gauge numbers remain the same, but there were no /1 sizes in between. so a gun proofed as a 13 could have been from .710"-.729" wath a 12 being .729"-.751"
The next thing of consideration is; at a point 9" from the breech the bbls are not really overly concerned about what the "MAXIMUM CHAMBER" pressure of the load is. IF you push a certain amount of shot out the muzzle to a given velocity then the same amount of work has been done, consequently, you will have a very close "Average" pressure over the length of the bbl. If one load has a significantly lower max chamber pressure then it will have higher pressure at some other points in the bbl to compensate.
Bottom line is assuming same overall balistics the load which would stress the bbl the least @ the 9" popint is almost certain not to be the lowest pressure load, but rather the one which had the highest max chamber pressure, as its pressure curve would fall quicker than would the one using a slower burn powder.
I am in agreement with HoJoe on this one, .025" at the 9" point doesn't have much of a built in safety factor. It would not concern me at all if it were 15" down the bbl.
Last edited by 2-piper; 02/13/13 07:28 PM. Reason: fixed a numerical error, year 4887 hasn't come yet
