Overbore formula for Intermediate cartridges

General Ammunition Discussion
#1

Hello guys,

This topic is a bit far from collecting ammunition, but I’ll still gonna ask since one must know that an intermediate cartridge overbore ranges between 4.29 and 7.99. As its formula says, overbore is equal to the internal capacity (volume) of the case in ml or cubic cm divided by the bore cross section area of the barrel in square cm. Finding the case volume is very easy, but the barrel bore cross section area not. Since most cartridges have this value under 1 square cm (ex. 0.4799 for 7.62x39), how can it be determined?
Thanks for any answers!

Bogdan

#2

The bullet diameter of your example cartridge is 7.9 mm (or 0.79 cm).
Divide this by 2 to get its radius: 3.95 mm
The cross section (area of a circle) then is: 3.95 x 3.95 x 3.14159 = 49.02 mm^2

From this cross section of the bullet we have to subtract the area of the lands in the barrel. William C. Davis recommends 1.5 percent as a rule of thumb. So the barrel cross section is 0.985 of the bullet cross section:
49.02 x 0.985 = 48.28 mm^2 (or 0.4828 cm^2)

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#3

I load 24 grains of powder under a 20 grain bullet in my 17 Remington.
I dont think it gets more overbore than that?

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#4

What were the bullet:powder weight ratios of the 7,92x94 and 7,92x107 rounds?

Ole

#5

Ok. Thank you for details. But I still have got some misunderstandings. Firstly, why did you have to subtract the area of the lands? Secondly, your result is 0.4828 compared to 0.4799 from the C.I.P. table sheets. Do you know what caused this error?

#6

MarchHatchet,
the area of the lands is not available for the powder gases to act on the bullet. The lands cut into the bullet cross section to make it spin.
The CIP area is the smallest barrel cross area that is considered acceptable for proof of a weapon. I frankly do not know how exactly CIP arrives at this figure. If you use the CIP area for all your calibers under consideration, the end result will be practically the same.

Chickenthief and Ole:
According to P.O. Ackley, “overbore” means that the case capacity is too large for the given barrel length to really make use of the propellant capacity available.
What you both refer to is the relation of propellant mass to projectile mass, which is not the same. I am not sure what the correct English translation is. For the 7.9 mm Patrone 318, propellant mass was 15 g, projectile mass 14.5 g. I have no figures for the Polish round.

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