Barrel thickness and pressures

[Pete Lester]

I found this on ShotgunWorld, if this is correct and it sounds logical, "thin" barrel(s) if it is 6 inches beyond the chamber are probably more of a dent hazard than safety hazard.

Following is the best explanation of pressure and velocity versus
length in a shotgun barrel that has ever been posted here.

Almost all shotgun powder burns within the first 2 inches
of the chamber. A typical fast burning powder will burn
in the first 3/4 inch of the chamber. This is where
maximum chamber pressure is attained. A typical fast
burning powder like Red Dot might give a chamber pressure
with a certain load of about 10,000 psi at that point.
This is why a chamber is very thick on a shotgun barrel
back at the chamber and gradually becomes thinner as
it gets further from the chamber.

After that first 3/4 inch, the pressure and tempurature
starts reducing. When the shot has travelled twice as far,
then the already burned powder which is now gas has
twice as much space available and the pressure therefore
drops to 1/2 what it was at 3/4 inch travel.

So when the shot has traveled 1 1/2 inches the pressure
drops to 5,000 psi if it was at 10,000 when the shot
had traveled 3/4 inch and was at max chamber pressure.

Likewise when the shot travels twice as far again, to
3 inches the pressure drops to half again. So when the
shot has traveled 3 inches the pressure is about 2500 psi.

This can be carried out on down the barrel and the
pressure keep reducing by half as the shot travel
doubles onward down the barrel. Here is a table giving
shot travel vs pressure.

inches:pressure

3/4 : 10,000
1 1/2 : 5,000
3 : 2,500
6 : 1,250
12 : 675
24 : 338

So as you can see as the shot travels further down the
barrel the pressure reduces. Since the pressure is
very high early on, the shot accelerates rapidly early
on. With 10,000 psi back initially it is accelerating
very fast. Once it gets out to 24 inches though the
pressure is only 338 psi, so the shot is accelerating
very little after that point.

[David Noble]

very interesting. who was the author? Was there any mention of the variations from this example when using the slower burning powders?

[Dean Romig]

Makes sense to me.

[Mills Morrison]

Very interesting. Makes a lot of sense. Hopefully, some of our experts on here will chime in and verify this.

[Eldon Goddard]

My experience tells me that the relationship with pressure and time in a highly dynamic event like combustion is almost never linear. I would need to study it further but that is just a first impression. This could just be a linear approximation, which there is nothing wrong with.

[Pete Lester]

Here are two pressure charts from ‘Powder Pressures at Different Parts of the Gun Barrel’ in the book ‘Sporting Guns and Gunpowder’, published by ‘The Field’ in London in 1897. Pressure drop looks pretty linear to me and seems to verify the post I found in SW.

http://i269.photobucket.com/albums/jj57/nhshotgunner/NitrowNotes001.jpg

http://i269.photobucket.com/albums/jj57/nhshotgunner/PressureCurve.jpg

[allen newell]

would this explain why quality damascus barrels can seem to handle the pressure of low power smokless loads?

[Dean Romig]

After about 6 inches from the breech the graph line seems to support the law of physics that states "an object in motion tends to stay in motion." but the high pressure in the first 2 inches is obviously the result of getting that "object" moving from a dead standstill.

[Mark Ouellette]

Quality Damascus such as used for Parker and other major concerns of the period are stronger than most think. Sherman Bell demonstrated his in this works for the Double Gun Journal.

[Pete Lester]

would this explain why quality damascus barrels can seem to handle the pressure of low power smokless loads?

Strike "can seem to" and you have it correct :)

[Bruce Day]

Strike "low" and substitute moderate and you have it correct.

But check your stock wood carefully.

1893 Parker catalog:

[Drew Hause]

The original article, and many more of interest
http://books.google.com/books?id=inQCAAAAYAAJ&source=gbs_navlinks_s

p. 32 Powder Pressures At Different Parts of the Gunbarrel, 1895
http://books.google.com/books?id=inQCAAAAYAAJ&q=pressure&source=gbs_word_cloud_r&cad=6#v=snippet&q=pressure&f=false
(scroll down and click on p. 31)

[Eldon Goddard]

You would think someone would be doing this work now. It does not seem that hard to do if you had the equipment. I am not sure about the accuracy of crusher gauges, but that was the best they had then. Not suprisingly higher pressure loads are less linear.

[allen newell]

Thanks Gents
Allen

[Drew Hause]

Have been working through the original document and the graphs Pete posted and came up with this.

32 Gr Ballistite (3 Dram Equiv.)
Breech - 2.9 Tons per Square Inch (UK, Long) = 6496 psi (pound force per square inch)
2 1/2" - 2.2 Tons = 4928 psi
6” - 1.2 Tons = 2688 psi
12” - .81 Tons = 1814 psi
18” - .33 Tons = 739 psi
24” - .21 Tons = 470 psi

83 Gr No. 4 Black Powder
Breech - 2.26 Tons = 5062 psi
2 1/2" - 1.96 Tons = 4390 psi
6” - 1.26 Tons = 2822 psi
12” - .9 Tons = 2016 psi
18” - .33 Tons = 739 psi
24” - .21 Tons = 605 psi

SO once again, the pressure is falling rapidly with either Black Powder or Smokeless and is quite low by 18"

[Drew Hause]

Turns out the conversion is alot more complicated :banghead:

Using the estimate Lead Crusher Pressure (Cp) conversion to PSI (pound force per square inch)
(Cp x 1.5) - .5 = TSI, TSI X 2240 = PSI

32 Gr Ballistite (1 1/8 oz at 1232 fps muzzle velocity = 3 Dram Equiv.)
Breech - 2.9 Tons per Square Inch (UK, Long) = 8624 psi
(2.9 x 1.5 = 4.35, 4.35 - .5 = 3.85 TSI, 3.85 x 2240 = 8624psi)
2 1/2" - 2.2 Tons = 6272 psi
6” - 1.2 Tons = 3718 psi
12” - .81 Tons = 1602 psi
18” - .33 Tons = 1120 psi
24” - .21 Tons = 694 psi

83 Gr No. 4 Black Powder at 1222 fps
Breech - 2.26 Tons = 6474 psi
2 1/2" - 1.96 Tons = 5466 psi
6” - 1.26 Tons = 3114 psi
12” - .9 Tons = 1904 psi
18” - .37 Tons = 1232 psi
24” - .27 Tons = 896 psi

[Eldon Goddard]

Does anyone know how high the thermal stresses are? What kind of temperatures are reached? Or are the thermal stresses small in comparison to the other stresses that they are ignored?

[Mark Ouellette]

Eldon,

The stresses are calculated as for a pressure vessel. Thermal doesn't seem to be a significant factor.

Mark

[Eldon Goddard]

Thanks Mark,
That is what I figured. It is to bad we did not have a chance to meet at the Las Vegas gun show. I would like the chance to speak more on these interesting subjects.

[Richard Flanders]

Didn't Sherman Bell produce some pressure curves with smokeless and BP curves? Anyone have them handy?

[Drew Hause]

The Double Gun Journal Summer 2002 "Finding Out for Myself, Part VI, Smokeless vs Black", p.19, and summarized in Volume 17: Issue 4, Winter 2006, p. 39

1 1/4 oz. 3 3/4 dram GOEX FFFg Black Powder at 1240 fps and equivalent load Blue Dot
..........1 inch.... 6 inches...... 12 inches
GOEX.. 5900 psi.. 4100...... 2100
Blue Dot 6000..... 4300...... 2300

1 1/2 oz. at 1236 fps Blue Dot (weight not stated)
.........10,000..... 4,400...... 2000