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Wall thickness

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Hi, Just wondering if there is a rule for wall thickness?
I heard about a rule of thirds where the walls must be at least the bore diameter- 1/3wall: 1/3 bore: 1/3 wall, but I can't find a reference for that.

Attached is a sketch of an original 19thC barrel (cast iron). I'm hoping to make an exact copy in bronze. Do you think the bore size should be reduced a little?
It's 36" overall, 2" bore.
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We have posted the rule from The North south Skirmish association in our stickies. Safe Loads & Construction-NEW CANNON...

It's not the rule of thirds, although that is what it amounts to. The rule calls for the wall over the breech to be one caliber thick. Caliber being of course the diameter of the bore. I highlighted the relevant section below.

This rule is for the construction of modern reproductions. You will find most original cannons of the era of the American Civil War. There are exceptions in original guns.

North-South Skirmish Association National Rules.
Actual or exact scale replicas of Civil War artillery pieces may be fired. The term “Civil War” applies to any artillery piece whose model antedates April 26, 1865. replicas of artillery pieces must duplicate original pieces. All reproduction barrels must be made of iron, steel or bronze. All reproduction barrels and those original failing inspection must be lined with a bore liner of extruded seamless steel tubing of a minimum ANSI standard and of a minimum 3/8-inch wall thickness.

Figure 10.1
The liner must be closed at the breech end with a steel plug, sweat-fitted into the liner and welded. The breech plug must have a radius of at least 25 percent of the bore radius and be at least 1 inch thick at its thinnest point. (See figure 10.1).
(N-SSA has advised us the term sweated means shrink fit.)

All reproduction barrels manufactured after March 1, 1986 must have pictures of the liner and breech plug before and after welding. No reproduction barrel shall be approved after March 1, 1986, that does not have one caliber’s thickness of metal surrounding the bore at the breech unless approved by Artillery Ordnance Officer. (See figure 10.2 for example.) Liner may be affixed by casting barrel around the liner or by other approved methods such as bonding with high strength adhesives. The method of locking the liner in the barrel shall be approved by the Artillery Ordnance Officer. The gun and its crew must pass the inspection specified in Section 18. A gun crew shall consist of a minimum of four members of the organization. Effective 1 February 1996, all artillery pieces must be originals or full-size, exact replicas to be approved.
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