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

I just began shooting a 10 gauge Damascus barrel SxS with 12 gauge Gauge-Mates (short sleeves).

Clearly the pressure (and "power") is significantly less than when firing the same load in a 12 gauge barrel. I'd like to understand the mathematics of calculating the likely pressure reduction percentage, due to the more spacious 10 gauge bore. I think it requires a calculation of the volume difference?.

If every other aspect remains the same, what is the likely pressure reduction of shooting a 12 gauge shotshell in a 10 gauge bore?

If someone can provide the calculation/answer, I would have some idea of how much to increase the powder charge in order to provide a similar pressure load; or a desired muzzle velocity. I'm using IMR #7625 powder in the 5,000 fps range.

Thanks for your assistance...

gold40

5000psi?

I am not and engineer, but I wouldn't think that there is any reduction in pressure, as a particular 12ga shotshell shot in a 12ga chamber, whether it is a 12ga gun or a 12ga chamber in a ten gauge gun, is pretty much the same, is it not? The insert only provides additional wall strength, to some degree, to hold that pressure.

Tests have been done that show there is no reduction in pressure when using gauge-reduction inserts. It seems intuitive that there would be a pressure reduction due to the increased bore volume as mentioned, but the tests did not bear that out. They did, however, show acceptable patterning results with these inserts.

The Ideal Gas Law is PV=nRT where P is Pressure V is Volume, n is moles of gas, R is a constant and T is absolute Temperature.
Assuming all else stays the same, the difference in pressure would be the ratio of the volumes into which the gas expands which would some cylindrical space, PiR2XL or Pi times the Radius squared times the length of the space. In this case it would just be the ratio of the Radius squared of the 12 gauge bore to the 10 gauge bore which works out to be 94% reduction.
One can threfore understand why a measuring device may show no difference.-Dick

Once again Folks there is a lot more to pressure than Max Chamber Pressure only. Obviously when you reduce the chamber to that of a 12ga chamber pressure would not be expected to change much. As it passes from the adapter into the bore itself pressure will drop below that of a 12ga bore.
Personally i would "NEVER" use 7625 at pressures of 5k psi, certainly not if temps might be expected to drop enough for a long sleeve cotton shirt to feel comfortable.

One can clearly see, hear, and feel significant difference in shooting this same 5,000 psi #7625 load in a 12 gauge vs, the 10 gugewith sleeves. My shoting partner coud readily see the slower shot string from the 10 gauge, but not from the 12.

By the way, the IMR loading manual lists 12 gauge loads with #7625 down to 4,200 psi.

Let me re-ask this question another way....What is the "volume difference percentage" between a 10 gauge 30" barrel, and a 12 gauge 30" barrel?

Thanks.

Jerry

jerry66stl:

What is your measured muzzle velocity?

Kind Regards,

Raimey
rse

The Area of Bore #1-12b is .4176 sq in or .4176 cubic inches per linear inch after chamber. The area of bore #2-10b is .4719 sq in/ .4719 cubic Inch per linear inch after chamber cone. Vol 2 is 13% larger / linear inch than Vol 1 therefore same gas volume would exhibit 13% less pressure. Thats the physics of it as it relates to gases. What may in fact be occurring is that the 12 b load/bore combination is at the low end of critical pressure(temp) to effect efficient combustion of the gases. Decreasing the pressure a further 10% or more may kick it over the edge resulting in incomplete combustion and a significant drop in pressure. Should you wish to attempt regain performance do not increase powder in anything more than 1 grain increments as pressure may rise quite quickly once the point of efficient combustion kicks in.

The ratio of the two radius squared times 100 (to get to percentage)

Berrien Moore