I have an idea what this means, what and how would it be determined for you

Are you refering to "Free Bore," like Weatherby is known for?

If you are, it is just a long throat that allows the Bullets to "Jump" before contacting the lands. This, in theory, allows you to use more powder for less pressure when compared to standard bores.

Reloader

In the context of cartridges, I think overbore is a larger case in relation to the caliber. Example a 300 Ultramag or 30-378 Weatherby is overbore as compared to a .308 Winchester. Not to be confused with the "freebore" of the Weatherby.

Lots of different opinions here, and I am not sure if there is an 'official' definition, or even who weould make it.

IMHO, 'overbore' means that the ration of case volume to bore diameter is such that no powder is available (slow enough) to approach casefull capacities. In that case, one cannot use the full capacity and potential of the cartridge, so it is 'overbore'. It is a moving target, too. For example, with the advent of powders slower than 4831 lots of formerly overbore cartridges become more useful...

I would have to agree w/ the others on the "Large capacity case, small diameter bore" Theory as well but, to some the terms "over bore" and "free bore" get mixed up.

Good Luck!

Reloader

I think when we speak of "overbore" cartridges, we are speaking of cartridges where there is a considerably greater powder capacity, in relation to the bore, within which the additional powder capacity reaches a point of deminishing returns. In other words the extra powder capacity doesn't produce a proportional increase in velocity.
Some cartridges which are considered overbore, are, but not limited to, all WBY MAGS, 25/06, all WSM etc.
A good example would be the 270Win/270WSM, where using H4831 you would use 60 grains in the 270 for a velocity of 3025 fps, and 67.0 grains in the WSM for a velocity of 3228 fps. 7 grains more powder for onley a 200 fps increase in velocity.

The definition given by CDH is that used by Ken Howell, former editor of Handloader, and is the only definition that really makes sense to me.

Yup. And it means as much as "too much." I.e., overbore to some may not be to others. An F-14 is very inneficient, but if you want that kind of performance, you have to pay the price.
Another definition would be "any cartridge witha great expansion ratio (powder to tube) than MY ideal cartridge."

Yes, Overbore was the topic.

I think my thoughts are in line with the general sentiment of what overbore is. I was wondering if there was a vast amount of concensus and documentated text on the subject.

The 270win/270wsm data points do not satifactorily point that one is overbore. 200 fps is not a gain I would dismiss. However, we would need a 3rd data point between these capacities that would yield close to 200 fps gain over the 270win with less capacity than the 270 wsm. But, like was stated, in the future another powder could take full advantage of the surplus capacity. The point where the capacity/velocity curve changes drastically for a given powder and barrel length is overbore for that situation for my understanding.