doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com A question about barrel pressure

Mid-1930s DuPont data validated by Sherman Bell "Finding Out for Myself, Part VI, Smokeless vs Black", in The Double Gun Journal - Summer 2002, p.19

I have saved a copy of the chart from Bell's article. The important points are that the peak pressures of both (BP and smokeless) powders occurs at about 1" from the breech and they both taper almost in a straight line, to the 12" mark. The loads are 3 drams of fffg BP and 1-1/8 oz. shot and same, using SR7625. I'll try to post the picture, later. Both smokeless and BP pressures follow essentially the same lines. Maybe that's why the barrel wall contours follow nearly identical lines, after 150 years??? The higher pressure of smokeless powder "out by where your fingers are" is total nonsense, as 2-p and others have said.

Thank you, Drew. While I was huntin' and peckin' you were gittn er done!

Thanks Drew, This may be a dumb question but just to clarify. The distance from piston to the breech in inches. Would be on a 2 3/4 inch chamber 3 3/4 inches from the end of the breech?

Jim, I think the concern "out where your fingers are" stems from two factors:

1. Where your fingers are is where your fingers are most vulnerable; and
2. While the pressure has dropped at the point you locate your leading hand, it's still relatively high (compared to what it is at the muzzle). And if you combine that relatively high pressure with a thin area in the barrel wall, maybe the result of honing or something of that nature, it can be a problem. If that thin spot is 3-4" back from the muzzle, not only is the pressure reduced, but your hand is some distance back.

Phillip, the issues you'll want to track down are Winter 01 (Long Shells in Short Chambers) and Summer 02 (Smokeless vs Black).

I know of 1 instance where a sxs shooter was useing chamber incerts from 12ga. to 20g. and put a walnut in the right barrel just where the forceing cone should be. The bulge was noticeable all the way around the barrel not just as a weak spot. The gun was a Holland and Holland not a piece of crap, the load was factory not a reload. That's all I know about it. : Mike

The standard point for taking the max pressure in a shotgun has for "LOOOONG" years been 1" from the "Breech". The end of a 2 3/4" chamber is 2 3/4" from the "Breech". The pressure point is thus inside the chamber, not beyond it. The above chart was preparred by E I DuPont De Nemours & Co, as best as I recall in the early years of the 1920's, long before Bell was evan a gleam in his father's eyes. The pressure from the 1" point to the 10" point is a decided & distinct "Curve", not a straight line. This is exactly what would be expected from the known characteristics of pressure curves from a propellant. Note also the two popints having the greatest spread occur first at the point of max pressure or just beyond the 1" ppoint & again at about the 4" popint. Also note the order of the various powders are exactly in reverse order at these two points. The curves are all crossing at about the 2½" point.
From this chart it can be seen the danger out where the "Hand" is is totally dependant upon the stated "Honing" of the wall to a too thin condition, but is irrespective of the type powder used. It thus has virtually nothing to do with the smokeless vs black issue as was so often stated for so many years, by those who should have done their home-work as this data was available.
Burrard stated (I believe most of his work was done in the 1930's, with some additions immediately following WWII) that "If" a shotgun bbl burst forward of the chamber or immediately forward of it, the chances of it being from an "Overload" were virtual non-existant. The most probable causes would be an obstruction or faulty metal (which could include being honed too thin).
It is well to note that when Bell did load increasingly heavy loads of "Smokeless" in both a damascus & a steel bbl'd Parker they both burst at very similar pressure (Very Excessive) & both opened up the "Chamber Walls", they didn't burst down where your forward hand holds it.

I have a test gun with a piezo strain gauge attached.It is 12 ga and I will insert a .410 tube and fire. The location is 1 inch from the breech and I will use factory winchester shells.Because of the added wall I think that the gauge will register much less strain on the primary barrel as I will not inform the computer (HAL)of the insert.Will take a while as I have some other projects

I would think a .410 insert in a 12ga bbl would have enough wall to completely contain the pressure & put virtually no pressure at all on the "Carrier" bbl. In fact I would highly susoect a .410 tube having even the length of the old Savage .410'er could be fired through a smoothed out bamboo tube opened to full 12ga size without splitting it.

I agree and will do .410,28;20 1nserts. I already know that the 20 is not stand alone and must have outer barrel support.So, I will report the stress on the outer barrel.Daggone,maybe Chuck H might try bamboo with inserts instead of Galazan for that Repro.Cool.