doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Pressure Question for Engineers....

Aren't you all assuming there is no gas leakage past the 12 Ga. wad in the 10 ga. bore? I don't believe it..

OB

Assuming nominal bore dias (.775/.729)² = 1.13 or 13% more volume for the 10ga. The quick & simple shortcut for volume of differing bore dias.
Yes I am well aware of the low pressure loads listed in manuals. Lets just say that from personal experience on a day I was breaking ice into a duck swamp (pre lead ban days) I experienced failure while using a load of 7625 taken directly from the then DuPont loading guide @ close to 7K psi, nowhere near that 5K one. I had worked too hard getting into that swamp to have it totally ruined by inadequate combustion, so "NEVER MORE" for this ol Boy. I will no longer load any powder slower than PB/Greendot etc lower than 7500psi & even then only with the hotter primers. These powders were designed for heavy shot loads at normal velocities with normal pressures, NOT for low pressure combustion. They get that quality by making them "Harder" to burn.
I can only state my experience, everyone has to decide for them selves, but if you are experiencing the differences you mention my guess is you are on the Ragged edge & any number of factors may give you a squib.

Agreed. I have switched to Clays for low pressure PRACTICE. My hunting loads (sadly used rarely...) are loaded with PB. I have personally shot PB loads down to temps in the 0*F range without problem. Any colder and I am going home anyway.

The approximations for pressure change calculated as a ratio of volume are OK for a general understanding where it is assumed that temperature of the powder gas is constant. However, the actual pressure would also have to include differences in temperature. Those who have worked with compressible fluids know that expansion of a compressed fluid causes a temperature drop. Clearly, the expansion between a 12 bore and 10 bore is different. Therefore, we would not expect for "T" to be exactly constant. I'd say that the actual change in pressure would be, directionally, more than the simple volume ratio indicates. Beyond this, it gets quite complicated; I will, however, be will to discuss if anyone really wants to.

I suggest to those that are interested -- please read the letter from Sherman Bell on pressure/velocity.
http://www.gaugemate.com/patterning.htm

I believe that one of S. Bell’s articles showed that the pressure peaks when the shot column has moved less than an inch and it is on its way down by the time the wad clears the shell mouth.
This would be within the insert, how much the insert adds to the barrel strength would be hard to say.

Regarding Mr. Bell's letter, I assume the reason that the pressure is greater in the 28ga shell in the 12ga bore vs the 28ga shell in the 16ga bore is that the pressure measurement was taken closer to the breech in the 12ga system. The temperature for the 12/28 tests was, however, 20 degrees (F) colder. Or am I missing something?

Berrien

Well let's see now we checked the Gaugemate 28ga adapter in the 16ga for pressure using Remington shells & measured "After" the forcing cone. We checked another adapter in a 12ga using Winchester shells with pressure taken "Prior" to the forcing cone.

With all due respect to Mr Bell he may as well have checked which would give the most milk, A Guernsey Cow or a Hampshire Sow. That test tells us a big fat absolute "0".

It has been well known for over a century that to insure that max pressure is recorded the sensor needed to be placed at about 1.0" from the breech. There was a reason that Gunmakers put the most metal "Over The Chamber".

2-Piper,

As noted in the letter, to put the strain gauge on the barrel wall over the adapter itself (as it would be at 1 in. from the breech) would give falsely low pressure readings because the adaptor would be absorbing much of the strain in that location, although I agree with your statement. The paragraph about the problems with locating the strain gauge in regard to the adaptor length explains this. It is a pity that there is no clear apples to apples comparison with regard to ammo brand and measurement locations.

However I believe, but do not know, that there is more to the story than in the letter. I discussed these tests with him over two years ago when I asked him the very question that the original poster asked here. He told me about the tests, but at the time was reluctant to have his name associated with them because it was contracted work for the manufacturer. I gathered that they might have been hoping to show a pressure reduction as well as good patterns, but were not able to, so the pressure discussion was intentionally absent in advertising literature. I also believe (but again, do not know) that he may have been personally hoping to show a pressure reduction for the benefit of the vintage gun enthusiast. I believe additional testing was done, but isn't in the letter because it wasn't useful to the manufacturer (can't show a pressure decrease) nor harmful (because they never claimed decreased pressure). I do know that his answer to me was a very direct "no" in regard to any significant pressure decrease. I have kept this information to myself until this moment, when for the first time I have seen at least some of the results published and signed by him. Given his straight-forward manner in answering me, and his usual thorough manner, I believe he probably has more data to draw his conclusion from than presented here, and likely would include at the least an apples to apples comparison of ammo from the same manufacturer.

Just my thoughts.

Another question about Mr. Bell's test--why would the pressure be greater in the 12/28ga combination than in the 12/20ga system? Or is the difference in the noise of his measurement system. I find this confusing at best, but it may be that I am dense.

I will appreciate anyone clearing the fog

Berrien Moore