doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Pressure differences 12 vs 20 bore

Hi guys,

Having followed lots of threads over the last few years, I suspect some of our American cousins will know the answer to my query:

Assuming a normal load of an ounce of bird shot propelled at about the speed of sound (1150 fps here at sea level) then is there any pressure difference between such loads fired in a 12 bore/gauge compared with a 20 bore/gauge?

My unscientific hunch is that the required pressure level may need to be higher in a 20 bore, but perhaps there is less friction in the smaller bore??

Here in Britain no-one seems to test or quote pressure levels. I have noticed that 20 bores SOUND quite different to 12 bores, but that could be different loads of course.

HB

William Bruette, Guncraft: Guns, Ammunition, Wing & Trap Shooting, 1912
https://books.google.com/books?id=5g51K93as84C&pg=PA210&dq
Pressures converted from long tons/sq. inch to psi by Burrard’s formula
Curtis & Harvey’s (42 gr. = 3 Dram) Amberite
12g 2 1/2” case 47 gr. (3 1/3 Dram) 1 1/4 oz. shot = 11,480 psi
12g 2 1/2” case 42 gr. (3 Dram) 1 1/8 oz. = 8,960 psi
16g 2 1/2” case 36 gr. (2 1/2 Dram) 15/16 oz. = 10,640 psi
20g 2 1/2” case 33 gr. (2 1/3 Dram) 13/16 oz. = 11,648 psi
Curtis & Harvey’s Smokeless Diamond (33 gr. = 3 Dram)
12g 2 1/2” case 39 gr. (3 1/2 Dram) 1 3/16 oz. = 11,984 psi
12g 2 1/2” case 33 gr. (3 Dram) 1 1/8 oz. = 9,632 psi
12g 2 1/2” case 33 gr. (3 Dram) 1 oz. = 8,288 psi
16g 2 1/2” case 28 gr. (2 1/2 Dram) 15/16 oz. = 11,480 psi
20g 2 1/2” case 26 gr. (2 1/3 Dram) 13/16 oz. = 12,320 psi

Ballistic Tables by DuPont c. 1920s
(Numbers are estimated transducer pressures)
12g
1 1/8 oz 3 Dr. Eq. 9000 psi
1 1/4 oz. 3 1/4 Dr. Eq. 10,250 psi
16g 1 oz.
2 1/2 Dr. Eq. 9000 psi
2 3/4 Dr. Eq. 11,000 psi
20g 7/8 oz.
2 1/4 Dr. Eq. 12,500 psi
2 1/2 Dr. Eq. 13,750 psi


Major Sir Gerald Burrard, The Modern Shotgun, 1955, 3rd Revised Edition Pressures converted from Long Tons/ Sq. Inch at 1”
Standard Load....................Standard Service......Max. Service
10g 2 7/8” 1 7/16 oz. 4 Dr. Eq. - 9,296 psi - 11,984 psi
12g 3” 1 1/2 oz. 3.57 Dr. Eq. - 9,632 psi - 12,320 psi
(1 1/2 oz. with 3 3/4 Dr. Eq. is 1260 fps)
12g 2 3/4” 1 1/4 oz. 3 1/4 Dr. Eq. - 9,296 psi - 11,984 psi
12g 2 1/2” 1 1/16 oz. 3 Dr. Eq. - 7,952 psi - 10,640 psi
12g 2” 7/8 oz. 2.36 Dr. Eq. - 6,440 psi - 8,960 psi
16g 2 1/2” 7/8 oz. 2 1/2 Dr. Eq. - 8,624 psi - 11,312 psi
16g 2 3/4” 1 oz. 2 7/8 Dr. Eq. - 9,296 psi - 11,984 psi
20g 2 1/2” 3/4 oz. 2 1/4 Dr. Eq. - 9,632 psi - 12,320 psi
20g 2 3/4” 7/8 oz. 2 1/3 Dr. Eq. - 9,968 psi - 12,992 psi
28g 2 1/2” 9/16 oz. 1 7/8 Dr. Eq. - 10,304 psi - 13,328 psi
.410 2” 5/16 oz. 7/8 Dr. Eq. - 8,288 psi - 10,976 psi
.410 2 1/2” 3/8 oz. 1 Dr. Eq. - 10,640 psi - 14,000 psi

All the modern pressures you'd want
https://www.baschieri-pellagri.com/en/catalogo/cartucce+da+caccia/1/1/index.aspx

Higher in the 20.
F=ma
F is force
m is mass
a is acceleration

To accelerate 1 ounce of mass to a velocity of 1150 requires a force which will be the same independent of gauge. Pressure is expressed as pounds per square inch. Force will be pressure (pounds per square inch) multiplied by cross sectional area of the bore (square inches). Since the cross sectional area of the 20 gauge bore is smaller, the pressure will have to be greater to get the same force.

The above considerations are the reason for SAAMI specifications for chamber pressure having a higher maximum allowed pressure for smaller gauges.

SAAMI speaks:



I usually don’t listen.

Best,
Ted

I don't understand why Reverend Drew had to go back to 1912 to find information that is available in 2025 loading charts. The answer is that "It depends on the individual load". At least, I think that is the answer.

If it's as simple as looking at a chart, or data from 1912, why can I load a 20 ga. load that achieves the same velocity, with the same payload, with less pressure than some equivalent 12 ga. loads?

I know and understand, the answer has to do with burning rates of different powders. So, wouldn't any accurate answer to HB's query have to include that information as a caveat? I did not understand him to be asking about max SAAMI pressures, but maybe I'm mistaken.

Bill & Stan get it - it depends on the powder and components.
The other point I was hoping to make is that:
1. load pressures have changed little from the smokeless powder loads of the mid-1890s, for which the U.S. makers proved their barrels in-house
2. the oft' said "Those old guns weren't designed for the pressures of loads today" is wrong. Esp. small bores, some pressures are documented to have been higher.
Smokeless Powder Co.'s “S.S.” (Smokeless Shot-gun) was a 43 grain = 3 Dram Eq. that was discontinued because small bore pressures were markedly higher.

My answer to the original question is an over-simplification in that it does not address the fact that pressure is not constant but diminishes with time and space, so it will vary with powders of different burn rates. If pressure could be kept constant in magnitude and duration (not the case with internal ballistics), a lower pressure would be required in a 12-gauge barrel than in a 20-gauge barrel to accelerate the same mass (1 ounce) to the same velocity (1150 fps).
The consideration of duration of the applied force explains how slower burning powders like Longshot produce higher velocity with lower peak pressure.
Still, I think that Pascal's law (Force = Pressure x Area) explains, at least in part, why SAAMI specifications allow higher pressure in the smaller gauges.
Disclaimer: I do not hold a degree in engineering or physics, but I did enjoy my introductory physics class, and I occasionally sleep in a Holiday Inn, although not last night.

I’m thinking I read an article on Chuck Hawk’s website, where he quoted a process engineer as stating they had changed only the primer used in a factory loading, and the pressure went up by 5000psi. What shocked me was they were altogether unconcerned about this, as long as it was under SAAMI max for the given loading.

I can probably find the article, but, when I said I usually don’t listen, that was one of many things that caused me to feel that way.

Best,
Ted

Primers can make a difference. Some time back I read an article by Tom Armburst on this question. They can make a big difference, 5k is unusually high. If you are loading low pressure loads and sub a primer for a similar one, giving same performance with the powder in question the difference shouldn't be of consequence, IN MY OPINION. So take that with a grain of salt.