I hear the term "overbore" used a lot. I've always assumed it meant a case diameter that's significantly greater than the bore diameter but you know what they say about assumptions. Anyone care to educate me with a definition?

From the Internet via Google search.....

There seemed to be some widely accepted, general rules of thumb as to what makes a case ‘overbore’. In the simplest terms, a very big case pushing a relatively small diameter bullet is acknowledged as the classic overbore design. But it’s not just large powder capacity that creates an overbore situation–it is the relationship between powder capacity and barrel bore diameter. Looking at those two factors, we can express the ‘Overbore Index’ as a mathematical formula — the case capacity in grains of water divided by the area (in square inches) of the bore cross-section. This gives us an Index which lets us compare various cartridge designs.”
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http://accurateshooter.net/Blog/7mmremmag1x350.png
OVERBORE INDEX Chart
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DOK - the images didn't come through.

I'd like to see the examples of which are considered overbore. I wonder why they would go volume to area instead of volume to volume. For example, in my 25-06 I could get the the same case capacity for signficantly different bullet weights, 60-120 grains yet the ratio based on the bore cross-section remains the same. I consider the light bullet a overbore example but not the heavy bullet.

Those are certainly interesting numbers, but they do not answer the question of WHY a given cartridge is or is not overbore - or WHY the dividing line is set at 1000 in the tables.

One common assumption is that it has to do with the powder burning completely before the bullet leaves the barrel. That's an incorrect assumption for two reasons: one, powders burn as completely as they are going to burn before the bullet moves even a few inches (some burn out before the bullet leaves the case!); and two, overbore has nothing to do with barrel length.

Historically, the unofficial term "overbore" referred to the volume of the cartridge compared to bore diameter (as defined above), but it had to do more with the types and burning rates of the powders then available. The rough definition was: if the case holds more of the optimum powder for that bore diameter, it is over that bore's required capacity: hence, overbore.

They didn't have the ultra-slow-burning powders we have today. At one time, the slowest powders available to reloaders were IMR 4064 and IMR 4320. Today, they are called medium burners! The 270, and many other such cartridges, hit maximum pressures with only about two-tirds of a case full of those powders. For those days, and with those powders, it was overbore. The .25-06 was even worse, being grossly overbore with those powders.

The change began to come with the release of war surplus 4831, which was dramatically slower. Suddenly, rounds like the 270 and 25-06 could be charged to 100% of case capacity without delivering excessive pressures. They were no longer overbore! But the only change was the powders available.

Today, the term probably has little if any meaning because we have canister powders hardly faster than charcoal briquettes. Even such ash-can-sized rounds as the 300 RUM or 30-378 Weatherby can be loaded to full case capacity with SOME powder(s), meaning they are not overbore.

Inefficient as all get out, but efficiency is another thing altogether. I personally define optimum efficiency as a ratio between case capacity and bore diameter, almost identically to that of the "overbore" definition above. Mine is a bit more simplified, however: Optimum efficiency occurs when case capacity in grains of water do not exceed the bore diameter squared, times 1000.

Note that it thus becomes a set number, and is irrespective of powder type.

Bore diameter Maximum case volume (rounded to the nearest grain of water)

.22 48
.24 58
.25 63
.27 73
.28 78
.30 90

and so forth...

Compare cartridge case volumes (especially classic cartridge designs) and you see that cases that do not exceed those numbers are invariably found to be powerful yet consistent, accurate and with broad application. Good rounds, all. Cartridges with cases larger than the number suggested are found to be finicky, erratic and of limited usefulness. Coincidence? I don't think so.

DOK - the images didn't come through.



Interesting, they are visible to me. Maybe clean living :):)

I got them after I left and came back to the site. Thanks.

Intuitively if you have a certain amount of energy released over a certain period of time from a confined space, there is a minimum diameter of an opening that can exist before the confined space fails(explodes). Of course the strength of the confined space comes in to play as well. I think this is what you guys are saying but looking at it a different way. So, the variables would be the total energy released, the time over which it is released, the cross sectional area of the escape opening and the "strength" of the container of the confined space.

Sorry...the engineer part of me is really geekin' out on this one!

As soon as you started your sentence with "Intuitively", we knew :cool:

My calculus instructor used that word a lot....he lied!!!!

Dan "brings back memories" OK

LOL!
Now, fugacity....I never could quite comprehend that one.

Also, intuitively is just another way of saying, " I can't do the derivation."

Overbore:
Definitions....
1: I don't like your cartridge
2: I don't think that is as practical as mine
3: Why burn all that powder for not much gain
4: Well maybe that is faster than mine, but you'll burn out a barrel first

Overbore:
Definitions....
1: I don't like your cartridge
2: I don't think that is as practical as mine
3: Why burn all that powder for not much gain
4: Well maybe that is faster than mine, but you'll burn out a barrel first

Now that's what I'd call fugacity.

I do love those overbore cartriges...
swift should be here soon, next rifle will probably be a
7mm mag... just because

_Kar.

Now that's what I'd call fugacity.

:DYou are killin' me dude! ROFL!