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

The red "ink" is for keeping direct responses separate from the original text. No meaning attached to the color. If this technique is offensive to you, say so and I'll not use it any more.

So are we to believe that just because the wad skirt of a 28 ga. wad can be flared out to .729" or so, it will still contain perhaps 9000 p.s.i. without that skirt blowing forward and losing its' perfect seal? Maybe not 9,000 psi. But the pressure has dropped considerably by the time the wad base must expand to bore diameter. That would be some tough skirt. Yes, the plastic in wads is very tough. Whew! I will still contend that even if that wad could contain the pressure, as the area almost instantly increased, the pressure would have to drop and force would remain equal (assuming zero blow by). I agree that this is very close to what must happen. Anything else would absolutely be claiming a physical free lunch. The only issue would be if the larger bore turned out to be a bit more efficient for the powder to burn/expand into. This could account for a small increase in wad base force and velocity. Are you saying that if we have, for example, 9000 p.s.i. gas pressure contained in a one cubic inch cylinder and we increase the volume of the cylinder to two cubic inches, that we would still have 9000 p.s.i.? Certainly not... not without adding more gas or superheating the same gas in the doubled volume. Now, I wish I knew how to put your following statement in red letters (highlite the text you wish to color, click on the "A" with a color bar under it in the bar above, and click on the color you wish) : "If the wad base expands to fill the larger bore, then, yes, the force increases. This is how a small hydraulic piston raises a large load on a bigger bore cylinder." Whiskey Tango Foxtrot. No, no, no. Huh uh. Nope. In a hydraulic pump, be it a simple hand Porta Power type pump (or a simple hydraulic jack), or a multi stage variable displacement rotary piston pump, the small diameter pump piston(s) driven by hand or motorized force displace a volume of relatively incompressable (or it could be a compressible gas) oil equal to the bore radius squared times pi times the stroke (length). This volume of incompressable fluid under pressure acts upon the base of the larger cylinder piston and moves it a porportionately shorter stroke. The larger the cylinder piston, the less one stroke of the pump piston will move it. Again, there are no free lunches here. Suppose we wish to lift 1000# with a cylinder having 10 sq in area. We will need 100 psi on the base of the cylinder (10 sq in X 100 psi = 1000 lbf). Suppose we have a pump piston with 1 sq in face area. Then we will need to apply just over 100# force to the piston to cause the fluid to flow to the cylinder and displace it. If we are unable to supply 100# force to the piston (1 in X 100 lbf = 100 psi), no fluid will move (the pump will stall). Pressure is constant within a hydraulic loop, but force is not at all constant. Your example is a fine statement of the displacement principle (second step), but assumes the pump has sufficient pressure to move the fluid. I working a step back where we have to know the pressure to force balance and area (first step). If the pump can't supply sufficient pressure, it will stall and no fluid will move. You have to work both steps. Lifting 1000# with 100# force seems like a free lunch. You pay for lunch with the displacement - sorta like spreading the payment out.

Jerry, I'd give you a simple, straight forward answer if there was one. As it is, I can only say that I see no reason for chamber pressure to be reduced, good reasons for barrel pressures to be slightly reduced, and MV to be reasonably constant. It could happen that the powder is not burning well in the enlarged volume (reduced barrel pressure) of the 10 bore barrel. You might want to try PB. Remember, as bore goes up, we need faster powder. However, we still must deal with keeping chamber pressure sufficiently low. I have a spiffy old Scott 10 bore hammer gun, so I'll be going through this same issue in a short while.

Excellent Discussion;
Now let me throw another pea or two in the pot. Assume the chamber insert is bored to exact dimensions of a regular 12ga chamber (it should be extremely close) then no gas volume has changed untill the base of the wad clears the end of the case. As max chamber pressure is normally reached & started to fall,prior to this point, no significant change in max pressure would be expected.
From the above quoted formula;

Note, This formula applies to "Total Volume" of the area containing the burning gasses. This includes the volume of the cartridge case as well as the bore it is entering. Thus iregardless of what the dia of the 10ga bore id in comparsion to that of a 12ga at the same point whether it be 6%, 13% or someother % that increase in volume is not immediately reached upon the charge entering the larger bore. The obturating cup of the wad would however immediately spring outward from the applied pressure to immediately give a larger area for the gas to push upon. Thus for at least some short distance after exiting the shell case the area to which force is applied would be greater than would be the falling off of pressure in a comparsion between the two gauges. Acceleration could then be expected to be greater in the larger bore, considering of course an adequate bore seal of the wads. "IF" that does not happen & blow-by occurs then all goes out the window.
Personally I have no inclination to try & calculate all the volumes necessary to come up with exactitudes. This would have to include inside case volume, volume of the forcing cones, & bore volumes. To get absolute definitve answers one would then have to figure changes in applied friction as well as change of the powder burn rate caused by the changing volumes.
Too much for this Ol Boy.

I was a consultant for GaugeMate for a number of years and was responsible for sending the adapters and ammunition to Tom Armbrust/Sherman Bell for testing. The primary reason for seeking their assistance was a question that had been raised by one of our clients.
I had done velocity (Oehler 35P) and patterning work with our 12 to 20-g adapters and was very confused by the velocity readings. The readings for 20-g Win AA (with a nominal velocity of 1200 fps) were usually around 1300 fps. As basically a 'rifle guy', I assumed that I was doing something wrong. The patterns were very uniform and dropping dove or powdering clay targets was no problem. In my 'rifle world' this was like using .25-06 ammo in a .30-06 and getting increased velocity and one-inch groups. It simply didn't make sense.
As a reader of the Double Gun Journal, I was familiar with Bell's work and decided that he would be a much more credible source for our client(s). His test results have been posted on the GaugeMate site for three years. Basically the results were that 20-g ejecta comes out of a 12-g barrel at a higher velocity than it does from a 20-g barrel. GaugeMate clients routinely reported similar results. Some of that feedback is also on the GaugeMate website.
About the same time, I visited the Federal Cartridge factory in Anoka, MN. I had the opportunity to discuss the 'velocity' question with Gary Svendsen who was then their Director of Quality and Product Service. Gary was not familiar with chamber adapters and said that Federal had not tested them. I explained the increased velocity readings we had experienced with 20-g ammo in a 12-g gun. He didn't appear surprised and suggested that the likely reason was reduced wad drag.
I hope this information is helpful. I should also note that I have not been associated with GaugeMate since 2007.
Steve Helsley

SH, thanks for jumping in with that information. Fortunately for shotgunners, the rifle analogies don't hold up. Shot is at least semi-fluid and plastic wads are very compliant. Ergo, they will fill a larger bore without large complaint. I think you were looking at a slight shift in favor of increased wad base force via enlarged wad base area vs lower pressure; the wad base area increased more than the pressure dropped. And, the powder found the larger bore burning conditions favorable/efficient. As for drag, HMWPE (wad material) has a sliding coefficient of friction on steel of about 0.02, as I recall, so it would take a very large change in lateral pressure on the wad base skirt and/or on the shot cup to have a significant effect on drag. The wad base skirts are going to see very similar internal lateral pressure in both barrels. The shot cup bearing length will shorten up by the volume ratio, say 6%. That just isn't going to account for 1200 fps to 1300 fps; a 17% increase in energy.

Rocketman, wouldn't it be that considering that lateral support will not be the same in a larger barrel, the wad will expand more in a "conic" way than in a "cylindrical" one (as it would in the same gauge barrel) and this would account for less wad base skirt making contact with the barrel and thusly less friction?

Could it also be, as it was mentioned, that a slight increase in internal volume will procure more oxygen that will help the powder to burn more efficiently?

This is, a 2P says, a great discussion.

JC

Good questions, JayCee!! Very thoughtful.

Thank you for the clarification, especially concerning the "equally well in a vacuum" part. Suppose the friction consideration is negligible.

I imagine extensive and expensive testing would be needed to really know what is going on.

JC(Always Learning)

Actually, I think some specific tests would give good insight. However, these tests have not been run - far as I know. However, having pooled information and knowledge, we do have a clearer picture than when this thread started. For now, we will have to live with what we have. But we do know much better what we need to learn in the future. Shotgun data tends to be general as there are significant differences form gun to gun.

Not to mention there are very few people with a vested interest in finding out the specifics we are interested in and deep enough pockets or the required equipment to run all the necessary testing.