I wanted to revisit something that got me thinking regarding the "one caliber" rule.
In my previous post titled "What to do with this thing" Dom responded with some interesting info concerning strength of barrel construction:
"Now to the Victor's "double sleeved" cannon tube. A cannon builder named Blakely in his writings, referred to a mechanical engineer named Barlowe. Barlowe found that the maximum strength in a tube was the inner portion of the tube. As you move toward the outer wall of the tube away from the center, that portion of the tube does less for strength. Another way to describe it is that more metal does not mean more strength. He found that there is a transition point in the thickness of the tube where you no longer add strength, just weight. The higher the tensile strength of the tube the closer the transition point to the inner wall of the tube. By shrinking tubes over the inner tube, a gun barrel's strength was increased a good bit over what it would be if it were one solid piece of the same thickness. This was the advent of the modern "built-up guns" in use today. Blakely, along with Treadwell, Parrot, Armstrong, Krupp and a few others, were original founders of this concept. I hope this explains the general idea without too much confusion. Dom"
What Dom noted here concerning the possibility of only adding weight rather than increased strength is valid. If the outer material of a barrel is not tensioned by heat-shrinking or some other method, it imparts less strength to contain pressure within the tube.
Let's look at a simple example...
Suppose we have a paper tube with a wall thickness of 1/16", closed securely at one end that we want to strengthen to withstand the explosion of a firecracker placed inside.
To best keep the paper from rupturing, should we:
1. Wrap the tube tightly with rubber bands to an additional wall thickness of 1/8"
2. Slip a close-fitting, (but not tight) rubber tube with a 1/2" wall thickness over the paper tube.
I think #1 would be the best choice.
So.... It is stated that a fairly thin-walled seamless steel tube (Which could not contain the pressure of a normal charge by itself), when inserted inside of an un-tensioned cast iron barrel, makes said cannon safe to fire if there is at least one caliber of thickness to the tube wall.
I guess it must be okay since there are so many built like this in use without problems. Maybe one caliber is enough to cover other strength related design deficiencies.
I wonder what forensic examinations of sleeved barrels that have failed have shown?
