Here is a chart, verifying what Mark stated, put out by Dupont showing pressure peaks in the first 1"-1 1/2" of the breech and then drops quickly as the load moves down the barrel. Many of us are shooting loads in our Parkers with pressures considerably lower than the loads that were tested by Dupont for this chart. I wonder if our nitro loads that develop ~4000-7000 psi behave similarly?

Greg,

Thanks for posting the barrel thickness diagram!

Toward what you stated, pressure in a contained space is constant. Since pressure is constant, the weakest (thinnest) area will rupture if the pressure is too high.

The Dupont pressure curve chart posted by Greg shows that as the wad and shot are pushed down the barrel the pressure is on a constant rate of decline. Key points of this are:

1. A load which creates excessive pressure will burst or bulge (cause permanent deformation) the area at or very close to the chamber.

2. A barrel that burst or split much past the chamber had an obstruction in it which caused a rapid rise in pressure. Since the wad and shot were far from the chamber the thinnest portion of pressure vessel (barrel between the breach and base of the wad) suffered deformation.

3. A very low pressure load can cause detonation which can damage a barrel. This is different than too high pressure caused by an overload or obstruction in #1 and #2 above.

Greg,

Yes they do. I have a peizo-electric pressure pressure gauge and my 6000 PSI loads have similar but reduced pressure curves as those measured by Dupont.

Mark

Chamber blowout

http://pic20.picturetrail.com:80/VOL.../346943274.jpg

More images here http://www.picturetrail.com/sfx/album/view/17546456

Is I understand it, Blue Dot Powder has the same pressure curve as black powder. The difference is that a much smaller quantity of Blue Dot is used to create those pressures. One of the principle reasons for using reduced loads in older shotguns is to protect the old wood. Even is the shotgun barrel has thick solid walls I would use a reduced load in an old gun.

A load can produce high recoil and low pressure or...

A load can produce low recoil and high pressure!

Excessive recoil will crack stocks while excessive pressure will deform barrels.

Mark, would you please explain this? What would be considered a very low pressure load in terms of psi?

Jent, the Frame Size is a "3". This is a "Waterfowl" weight, heavier frame, heavier barrels. Don't know the barrel weight yet, but supposedly heavier than typical. Once I can determine status of barrels then maybe I have something worthy of shooting. Granted, I could alwasy put in some Briley Tubes and shoot 16ga or 20ga but I think I'll hopefully be able to shoot this one with some Vintage shells.

Greg,

This is mysterious and I do not completely understand the causes of detonation is a shotgun or pistol. I am not certain anyone does...

Detonation is an explosion rather than the deflagration (buring) of power. High explosives detonate via a supersonic shock wave. For low explosives such as smokeless (and black) powder, maybe under under rare circumstances a squib (far too light) powder charge may cause detonation which is otherwise known as a Secondary Explosive Effect?

Detonation may occur when using a squib load which is a serious concern when loading some light powder charge revolver ammo. I have not encountered low pressure shotgun data which produces much less than 6000 PSI in a 12 gauge. Okay, there may be a load or two at 5000 PSI but they are few and far betweeen...

To cause a detonation... If one would try to develop loads at much lower pressure than 5000 PSI they may cause a detonation which might burst or bulge a barrel at the weakest point between the chamber and the base of the wad. Perhaps a shock wave may result when the powder is mostly burned and the wad is still traveling down the barrel creating a vaccumn?

Another theory is that a reduced powder charge might be ignited too fast by the primer. If burn rate is determiend by the surface area of a powder then direct explosure of too much surface are at the energy of a primer might induce a faster burn rate? Just a theory of mine...

Where are the explosives experts when I need them? :)

That's my best for now...
Mark