doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Gas Pressure at Muzzle of Shotgun Barrel

Does anybody know what this typically runs or how to measure it? If you've got say 8,000 psi on ignition, what does it drop to at the muzzle? Can this be calculated if you know the max chamber pressure and barrel volume? If so would you have to know exactly where the shot charge is in the barrel when max pressure is reached? i.e. a few inches either way should make a large relative difference in the calculation. I don't know if the usual strain gage setup would work as the radial pressure from the shot hitting the chokes would tend to skew results. Thanks for any help. John C.

Pressure drops to a fairly low value at the muzzle, say 4-500 psi. Peak pressure occurs in the chamber before the shot has traveled an inch. Winchester published tables and graphs long ago that may be found in Butler 'The American Shotgun'.

Calcualating muzzle pressure from peak chamber pressure would not work as there is additional powder burn (gas generation) after peak pressure is reached, I'd expect. If you had a big enough computer and knew enough of the powder characteristics, you could calculate it. I don't know any practical calculation from what is available.

The pressure is quite low, I'd guess more on the order of 50-200 psi - Greener is reported as turning a section of a barrel at the muzzle to near "foil" thickness and firing without a burst. Further, he is reported to have removed the thin section with a pen knife after the demonstration.

I would expect a strain gage set-up to work just fine. IMO, the shot doesn't "hit the choke." Shot is acting as at minimum a semi-fluid and "flows" through the choke. I expect the shot column has an internal pressure that is a very near reflection of the gas pressure behind it and it is subject to pressure reduction and velocity increase in a constricted passage (venturi effect) such as the choke. Note that a constriction such as the forcing cones or chokes is not to be confused with a restriction/obstruction such as a foreign object inside the barrels. I'd expect a circumferential orientation of a strain gage to yield very accurate pressure within the choke.

This is a subject of considerable interest to me. Let me know if you want to work on it.

Source 1
Article in the (UK) The Journal of The Forensic Science Soc.
Auth. Victor Drake
Units Brit Long Ton (2240#/T) Radial Lead Crusher Method
Max 2.8T @1"
2.5T @6"
0.5T @14" Linear decline hereafter
0.25T@20"
0.1T @30"

Source2 Burrard The Modern Shotgun , Ch. Pressure
2.8T@1"
0.36T@20"
0.2 T@30"

Both typical Brit Flake Powder 2&12",12b., 1&1/16oz loads vis Eley Grand Prix etc.
Range 220-450 psi.

N.B. Progressive or slower burning powders would increase this e.g. Goose Loads with Blue Dot etc.
Hope this helps
Hugh Lomas

Guys thanks for the input, been away for a day or so. I'm really suprised the pressure is as low as you say. Only 250 to 500 psi down from 8000. Whats precipitous drop do to, can't just be increase in confinement dimensions by the addition of the barrel volume, is there a temperature drop involved as well as the gas moves down the barrel? Rocketman, I am also interested and will keep looking and let you know if I find anymore info. John C.

Not being an engineer I can only give you a logical answer: The drop IS due to the increased volume of the entire barrel and the fact that the burning is nearly over, as is the expanding of the gasses, IMO. Think how low the pressure is an inch out of the muzzle! Zero! I don't know why that's surprising.

Consider how little volume is in the shell combustion chamber to start with, how little more is added by the wad moving one inch (about the movement at pressure peak), and you begin to see the expansion ratio. Yep, the pressure drop is due to expansion. We would achieve some real screamer MV's if the pressure stayed up; Pressure times wad base area equals the force on the shot load and acceleration of the shot load equals force divided by mass. Expansion of a gas is usually associated with a temperature drop; both get accounted for by the pressure drop.

I doubt that slow burning powders have a lot of influence on muzzle pressure; some, but not a lot. If they did, I think we could use them more to tune patterning/choke effect much more that we are currently able to do.

From my collection: W-W lab data on three, different 12 ga factory loads: 700 psi @ 26"; 620 psi @ 28" and 605 psi @ 29".

Just for grins, a model that assumes PV = constant (a slightly pessimistic or high assumption) indicates that Sherman Bell's 12 ga, 1 1/8oz Bi, smokeless-for-black load would have shown 700 psi at 30", had Sherman wasted a pressure transducer there.

Considering calibration issues (fraction of full scale and all that) I wouldn't bet on these being very precise -- maybe +- 100 or 200 psi????

This explains why Greener was able to hone a set of barrels out until they were paper thin and they never failed. He destroyed the barrels by cutting them up with a pen knife the stroy goes.

Consider that the barrels taper down to the .030 wallthickness range in only about a foot or so. Pressure can't be much even in the middle of the barrels.