A few more thoughts:
First off, the value people see for Tensile strength is always a really big number like 66,000 PSI and they get the idea that a powder charge is safe as long as less than 66,000 PSI exists in the breech of the barrel. This is not the way it works.
When the value is given it applies to the crossectional area in square inches of the material restraining the load.
Lets consider a simple rod of a known material and lets say it has a tensile strength of 66,000 PSI.
If that rod was 1/4 inch in diameter, any crossectional cut thru that rod will have an area of pi X R squared or, in this case, .0491 square inches. This is the square inches the value is referring too.
Using the .0491 square inches times 66,000 PSI gives a value of 3,239.76 Pounds, so, the when the rod is subjected to that much pulling load you can expect it to break. Tensile strength by the way is the breaking or failing strength of the material and all good designs stay well away from even approaching that value.
Repeated applications of pressure weaken the material. Rapid applications of pressure such as firing gunpowder rapidly reduce the strengths which explains why the safe working pressures are greatly reduced in all Engineering calculations. In the case of the calculations below I am using the suggested value of 1/10 of the calculated burst pressure.
Getting to the real reason for this post, I sometimes hear folks talking about using common pipe to build a barrel from. They sometimes say, "I found some Schedule 80 or Schedule 160 pipe with really thick walls and I'm going to build a barrel with it."
I think I should talk about this a bit.
First off, regular pipe is always made by rolling a flat plate into a tube and welding the ends. This may be a butt weld or a lap weld and pipe made in this manner has the same built in weakness as welded tubing.
Regular pipe used to be made from iron but now is usually made from the cheapest steel that can be found. That is because, generally speaking, pipe is intended for relatively low pressures.
Looking thru a "TUBESALES" catalog there are several different Schedules of pipe available.
The typical Schedule 40 that's used in household water and gas applicatons, Schedule 80 that is used for moderate pressures, Schedule 160 that is used in fairly high pressure systems and a Schedule XX STRONG for special applications.
It should be noted that in all of these applications a rapid application of pressure is never considered.
Using this catalog I decided to run a few numbers for a shotgun barrel made from the Schedule XX Strong.
They show a "1 inch" pipe of this Schedule which has a .358 wall. The outside diameter is 1.315 and the bore is .599 (a little smaller than a 20 guage).
A well known source specifies a tensile burst strength for all plain pipe as being 40,000 PSI for a butt welded pipe.
Using Barlow's Formula for calculating safe vessel (container) pressures which is "P = (2T X S)/D" where T is the wall thickness, S is the tensile strength of the material and D is the outside diameter of the vessel.
For this XX STRONG pipe, the numbers look like this:
P = [2 X .358 X 40,000]/ 1.315 = 21,779.47 PSI This pipe is not safe at this pressure.
It must be noted that this is the ultimate pressure at the time of failure and the working pressure is specified as being only 1/10 of that value. In this case, the working pressure would be 2,178 PSI.
Knowing that almost no one will be able to find some Schedule XX STRONG pipe, I then ran the numbers for some Schedule 160 pipe.
I found some "3/4 inch" Schedule 160 pipe that has an outside diameter of 1.050, a wall thickness of .218 and an inside diameter of .614 (about a 20 guage).
Plugging in the numbers gives:
P = [2 X .218) X 40,000] / 1.050 = 16,609.5 PSI This pipe is not safe at this pressure.
1/10 of that value to obtain a safe working pressure is 1,661 PSI.
These "working pressures" by the way are the maximum pressures I would say are safe.
Unfortunately I do not have any data for a 20 guage shotgun however I do have data for a 12 guage (Lyman Black Powder Handbook).
For a 26 inch modified choke barrel Lyman shows the following pressures:
2 3/4 dram (75 grain) & 1 1/8 oz # 7 1/2 shot =2,700 PSI with GOEX 2Fg powder.
3 dram (80 grain) & 1 1/2 oz #4 shot = 3,600 PSI with GOEX 2Fg powder.
I might also point out that usually the smaller bore guns have greater pressures than the larger bore guns.
It should be noted that both of these 12 guage pressure values exceed the safe working pressure of even the Schedule XX STRONG pipe so it is obvious that the Schedule 160, 80 and 40 pipe are to be considered dangerous for use as a barrel making material.
Although it is easy to say, "Well old zonie is just covering his butt with that ridiculous 1/10 value. I can use 50 percent and be plenty safe."
remember, by exceeding the calculated working pressures you are subjecting yourselves and those around you with DEATH or worse. Worse includes being blinded, paralyzed or loosing major parts of your face. This is not an area that people who don't understand Engineering principles should mess around with.
Enough of the Engineering for now. My head hurts. :thumbsup:
