Pressures in tube construction--Why 2x's times the bore?

[powder2burn]

Have some questions on tube (cannon barrel) construction.

A. Switlik's pressure charts in "Artilleryman" Vol. 9 #4 p 8 & 9, showed the highest average pressure attained as 26,300 psi. (3" ordnance rifle)

B. If  modern day metal tubing can withstand pressures  1 i/2  or 2 times the 26,300 psi, and the thickness of the tubing is less than than 21x's the bore, why do you need 2x's the bore, for the chamber or breach?  I'm not talking about the breach plug.

[Cat Whisperer]

A. The pressure is not static - merely reading the pressure it will withstand is a simplification - especially over time.
B. There are corrosive elements of combustion.
C. Design also plays into the equation - rounded inside corners to reduce stress risers.
D. It is a PROVEN by experience principle - from organizations as N-SSA and AAA - that works.
E. IF it is excessive it is safer.

There are 5 reasons.  Designing firearms starts with planning and the designs are then TESTED.  Some pass, some don't.

[Double D]

Two  national Organizations with years of shooting experience and some highly knowledgable people have settled on a SAFETY Rule of barrel wall thickness of 1 caliber for muzzle loading cannon. Why would you want to challenge that.  I know as a moderator I would not want to advise anything other than that.

With some modern steels you probably don't need that much.  There are to many varibles  and it much simpler to stay safe  with the proven rulle.

[GGaskill]

The first thing you need to consider when seeing numbers such as 26,300 psi chamber pressure is that materials have to resist stress, not pressure, even though the units are the same.  A chamber pressure of 26,300 psi can induce a stress of 26,300 psi or a stress of 100,000 psi depending on the chamber diameter and wall thickness.

There are a couple more things than just the chamber pressure and yield strength of the barrel material to have a safe design.  One is fatigue failure and the other is safety factor.

"Fatigue" is the traditional name for failure due to repeated application of load, such as firing a gun.  The normal practice in steel is to allow twice the strength of the static load stress to reach a point where fatigue failure will not occur, which means if your calculated static load stress is 30,000 psi, then the material strength should be 60,000 psi.  Now in a typical gun, you are not going to reach the 10,000,000 repetitions that represent the no fatigue situation, so you can build to a lesser standard, since the barrel will wear out before it fails in fatigue, but you need to do the real, not backyard, engineering to be able to confidently reach that point.

Safety factor is something that is used to compensate for things that cannot be readily determined, such as exact loads, exact material strength, etc.  It is different for different structures--airplanes have small safety factors as large ones would prevent flight.  This is compensated by frequent inspections and a schedule of part replacement before the expected failure time.  Other structures have large safety factors and don't require special handling, although some inspections and repair as necessary do occur.  Safety factors for guns tend towards the smaller side because they need to be mobile to a large degree.  Their relatively short life span allows operation in the lower end of the fatigue range since no gun reaches a 100,000 cycle life.

Anyway my conclusion is that you should stick to the chamber wall = chamber diameter rule unless you are qualified to do the engineering (and do it) or engage a professional engineer to do the engineering.

[Cat Whisperer]

The first thing you need to consider when seeing numbers such as 26,300 psi chamber pressure is that materials have to resist stress, not pressure, even though the units are the same.  A chamber pressure of 26,300 psi can induce a stress of 26,300 psi or a stress of 100,000 psi depending on the chamber diameter and wall thickness.

There are a couple more things than just the chamber pressure and yield strength of the barrel material to have a safe design.  One is fatigue failure and the other is safety factor.

"Fatigue" is the traditional name for failure due to repeated application of load, such as firing a gun.  The normal practice in steel is to allow twice the strength of the static load stress to reach a point where fatigue failure will not occur, which means if your calculated static load stress is 30,000 psi, then the material strength should be 60,000 psi.  Now in a typical gun, you are not going to reach the 10,000,000 repetitions that represent the no fatigue situation, so you can build to a lesser standard, since the barrel will wear out before it fails in fatigue, but you need to do the real, not backyard, engineering to be able to confidently reach that point

Safety factor is something that is used to compensate for things that cannot be readily determined, such as exact loads, exact material strength, etc.  It is different for different structures--airplanes have small safety factors as large ones would prevent flight.  This is compensated by frequent inspections and a schedule of part replacement before the expected failure time.  Other structures have large safety factors and don't require special handling, although some inspections and repair as necessary do occur.  Safety factors for guns tend towards the smaller side because they need to be mobile to a large degree.  Their relatively short life span allows operation in the lower end of the fatigue range since no gun reaches a 100,000 cycle life.

Anyway my conclusion is that you should stick to the chamber wall = chamber diameter rule unless you are qualified to do the engineering (and do it) or engage a professional engineer to do the engineering.

Agree with the conclusion.
Let me say also that if you copy an existing proven design and load it responsibly you'll have a 'safe' piece.

ENGINEERING a design considers ALL the aspects of design, materials and loading. 

As I've said before, I've seen a cannon explode from about 15' away.  You've GOT TO KNOW if it's a cannon or a bomb when you light the fuse.