The term overbore is used in discussions of the likes of 7mm STW and others....please explain this to me.

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Originally posted by ted thorn:
The term overbore is used in discussions of the likes of 7mm STW and others....please explain this to me.

Basically the cartridge has become inefficient in terms of quantity of powder burned to velocity produced at the muzzle in that you can only drive a bullet of "X" weight in a bore of "Y" diameter so fast.

So that a stage is reached where the more powder you add you don't increase the velocity by that same percentage that you have increased the powder charge.

So whilst a 280 Remington gives a significant increase in velocity over a 7mm Mauser a 7mm Remington Magnum does not give as big an increase in velocity over the 280 Remington. And the 7mm STW even less of an increase in velocity over the 7mm Remington Magnum.

It means that the right powder hasn't been made for that chamber capacity to bore ratio. roger

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Originally posted by enfieldspares:
Basically the cartridge has become inefficient in terms of quantity of powder burned to velocity produced at the muzzle in that you can only drive a bullet of "X" weight in a bore of "Y" diameter so fast.

So that a stage is reached where the more powder you add you don't increase the velocity by that same percentage that you have increased the powder charge.

So whilst a 280 Remington gives a significant increase in velocity over a 7mm Mauser a 7mm Remington Magnum does not give as big an increase in velocity over the 280 Remington. And the 7mm STW even less of an increase in velocity over the 7mm Remington Magnum.

A pretty good reply!

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It means that the right powder hasn't been made for that chamber capacity to bore ratio. roger

Interesting way to put it.....do you think the right powder will ever be developed to do this?

in basic terms, the case is too big for the bullet diameter
an example would be a 50 bmg case necked down to a 30 caliber bullet.
the bore diameter of the rifle will be too small to burn all that powder

And this all means squat without a major dose of context.
the .284 caliber is a great example. The notion of overbore revolves around efficiency.
If you can get a M70 chambered in 7mm STW for $800 from someone who doesn't know better, and procedeed to kill every elk or sheep you get a chance to, with no regrets or hesitations about the shot you're presented with, then there you have it.
To burn a certain amount of powder you need a commensurate amount of bore (hole size and tube length). If you don't have enough tube you just make light and heat instead of speed. "Overbore" assumes a 24-26" bbl on a hunting, as being the "reasonable" length when in fact there are a number of 30-34" tubes pushing the heavy-for-caliber bullets out of the larger cases.
Roger's comment has merit as well. The main complaint about overbore cartridges may not be that they simply eat up powder, but that they also eat up throats. This is more due to temperature than anything else. Slower powders are able to flatten the pressure peak and can do a lot for throat life.
Bottom line: find the gun that does what you want and forget the term "overbore." You will never get F-14 performance from a Cessna, but likewise you don't need an F-14 if all you want to do is get in the air.
That said, there's something more satisfying to flattening a whitetatail with a 6.5 Swede than with a 300 RUM. There just is.

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quote:

Originally posted by bartsche:
It means that the right powder hasn't been made for that chamber capacity to bore ratio. roger

Hey Bartsche, Excellent answer which does not confuse the issue.

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Originally posted by vapodog:

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It means that the right powder hasn't been made for that chamber capacity to bore ratio. roger

Interesting way to put it.....do you think the right powder will ever be developed to do this?

Not in our life time! roger

So has anyone ever come up with a chart that would list appropriate case volume per bore diameter and barrel length?

The concept of overbore deals with two issues: Case capacity and barrel volume. For example: If one compares a 270 and 30-06 in 22 inch barrels, since the volume of the 270 barrel would be less than the 06 barrel and the case capacity is essential the same, one could describe the 270 as more overbore than the 30-06. The only way you could make them equal would be to increase the barrel length to make the 270 have the same volume as the 30-06. In similiar fashion, one can increase the barrel length on a 7 STW until it had the same relationhip of case capacity to barrel volume as say a 308, which is considered a highly efficient round. I don't know what length that would be, but a little simple algebra could solve it pretty easily.

Why hasn't Hornady fixed this overbore problem with some of that magic powder they use?

I heard that the 35 whelen is one of the most efficiant cartidges developed.

Overbore is a term that was coined to allow firearms writers to take up space in magazines. It also allows some folks to feel better about themselves for shooting "sensible" cartridges. You'll hear statistics and terms like efficient and the like, but the best one is that overbore rounds cost more to reload. This one really gets me. I have read several articles about the number of rounds per pound of powder and crap like that. Man, if I have to get down to worrying about 5 grains per round, I'll quit.

I hear you, MH1.
And ideas like Alf's just perpetuate the problem. I mean, can anyone actually read that post straight through?!
"Overbore" means "over" something someone says is "ok." If you buy into that guy's definition then all's well and good. But the notion of "acceptable" in terms of velocity gains per grain of powder in a given SD bullet are simply some group's idea of what "ought" to be.
The 6.5 Swede and those along that same curve offer great velocity:SD ratios. But that isn't always what you want. To repeat, it has much to do with barrel length as well, since it is all about the ability to burn the powder before spitting out the pill. A man that doesn't mind a 28" tube on his gun cannot effectively use data about "overbore cartridges" when that data is centered around a 24" barrel.
A very simple way to put it is, find a bullet you want to push, figure how fast you want to push it, find the case that does it with eas within the gun parameters you're comfortable with (action length, barrel length, etc.) and accept the compromises in life.
And if there's two cases that do it, pick the one that does it at a lower pressure (not less powder necessarily.)

Powder that will work better as far as efficiency is concerned for some over bore cartridges is around it is in the form of medium caliber cannon propellants and a wealth of burn rates is not made, it is also hard to get unless you know the rite people. I am so addicted to wildcats that I have incorporated small batch powder manufacture in my 10 year plan. Weather or not it will be MORE commonly distributed is up to the market it will however never even be know to all handloaders.

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Another way of referring to over-bore cartridges is to talk about BARREL BURNERS. That way we do not have to solve agebriac equations as what length of barrel we ought to have or to wait for another breakthrough on a new powder that will revolutionize the current problem that we face, which is the erosion of throats, by essentially using lager cases (more powder) in smaller bore rifles and so create higher flame temperatures over a smaller bore surface. Extending a barrel from 24" to say 28" is not going to solve this problem.

Warrior

quote:

Originally posted by Magnum Hunter1:
Overbore is a term that was coined to allow firearms writers to take up space in magazines. It also allows some folks to feel better about themselves for shooting "sensible" cartridges. You'll hear statistics and terms like efficient and the like, but the best one is that overbore rounds cost more to reload. This one really gets me. I have read several articles about the number of rounds per pound of powder and crap like that. Man, if I have to get down to worrying about 5 grains per round, I'll quit.

roger