I also have real concern when I see holes drilled in the breech "business" end of a cannon. If these 3 holes were drilled through to the bore, then that would really weaken the cannon. I would still be a little nervous having 3 extra holes bored through only half of the wall thickness of the breech.
As an engineer, I have to look at the stress analysis of this "modification". The cannon is classified as a Thick-walled Pressure Cylinder, meaning it is a pressurized cylinder that has a wall thickness that has a large ratio of Wall thickness to Cylinder Outer Diameter. The stresses that build up on a pressure vessel will be different for the different wall thicknesses.
I worked on aircraft Landing Gears and I didn't understand these struts(basically a hydraulic actuator) as a cylinder. It wasn't until I observed a test that used polarized light and a plastic coating that allowed you to actually see the stress patterns. Under pressure, the cylinder walls expands like a balloon. And these were steel cylinders that have a 1/4 inch, high-strength(Rc=52) wall.
I found this stress analysis of a thick walled cylinder that shows a Computer Model(FEM) of the stress patterns that would match what a cannon would be exposed to during firing.
Some of these stresses are for a closed cylinders---as the pressure expands, it causes the cylinder to grow longer. This would not apply to a Cannon that has an open breech. Also, the computer model looks at principal stresses which are stresses going in one direction.
FEA of Thick-walled Cylinder
Some images from that paper.
Notice in the Left image, the stresses are in the YELLOW range at almost half the wall thickness. If a hole is drilled into this stress zone, then this acts as a stress multiplier (aka stress concentration factor).
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