i hear this term used a lot and i have 3 questions relating to it.

1. what does it mean?

2. how can i tell if a cartridge is considered overbore?

3. i once saw a formula that was used to determine the theoretical maximum amount of powder to use for a specific caliber. i think the idea was that at this powder weight the cartridge was considered to be on the line between overbore and not. does anyone know what this formula is?

thanks for the help....

Bill Smith
There's lots of guys and gals better qualified to explain overbore.
Let me give it a shot. In a 22 rim fire a 15 inches barrel burns up all the powder and the maximum velocity is reached. I think the remaining barrel actually slows the bullet down.
Ok in a 25-06 a 26 inch barrel is just about right the powder is burned and the bullet will not accelerate any more or at such a minute degree as to be insignificant. This same caliber at 18 inches blows a lot of powder out the muzzle. This rifle, with the 18 barrel, is over bore, the amount of powder needed to influence the velocity is out of proportion to any gain.
Over bore can be influenced by the length of the barrel, the speed of the powder.
Using the the same 25-06 with the same bullet weight, Bullseye will be over bore real quick. H870 will not reach the optimal pressure with the 06 case in a 25 caliber barrel.
Now that I've muddied the question, lets read all those folks who know a whole more then this guy.
Jim

If you neck a 50 BMG down to .224, you will not get a gazillion feet per second.

For every diamter bullet there is a case capacity that gives maximum muzzle velocity, or close to it. If you make the case larger than that, you get very little more speed, a lot more barrel wear, the load becomes very finicky, and we call it "overbore".

arkypete: good explanation. thanks.

denton: " For every diamter bullet there is a case capacity that gives maximum muzzle velocity, or close to it."

now.......do you know the formula for determining that case capacity??

The term overbore capacity means that the cartridge can hold more powder than the bore can handle efficiently
There is no way to determine max case capicity.

If you could find a copy of Handbook for Shooters and Reloaders by P O Ackley there is a chapter in vol. 1 that explains it very well. This is a two vol. set that has a welth of information in it.

So, understand the term "overbore" is a lot like the term "too much."
There is an performance/efficiency curve for any bore diameter, and you just have to find where you feel comfortable.
The 35 Remington is MUCH more efficient than the 35 Whelen, but you don't hear people calling the Whelen "overbore."
The 308 is much more efficient than the 300 RUM, but many feel that the downsides (longer barrel, louder report, faster/stronger recoil, more expensive powder charge) are overcome by the speed with which the bullets leave the barrel.
If you want to figure out which cartridges are considered overbore, I suggest the words of Bob Forker: "Overbore is a term a cartridge designer uses for the cartridge a competitor has designed that is larger than his biggest offering is. If Mr X designs a cartridge that is larger than Mr Y's largest design, then it follows directly that Mr Y will call Mr X's new cartridge "overbore," or even "grossly overbore."
He goes on to explain expansion ratio, (overall volume of barrel divided by useable volume of cartridge case) and to suggest that a ratio of 4 is perhaps the lower limit of practicality (e.g., 30-378 Weatherby w/24" bbl) and that 6-8 are good compromises for sporting cartridges. A 30-06 w/24" bbl has an ER of 7.
To get a higher ER, just lengthen the barrel. According to this guy, once you get to about 50, you actually begin to lose velocity, since the powder has done all it can, and you're just creating friction. This happens with this 30-378 Weatherby right at about a 400" bbl.
So you see, it really is just a matter of how much recoil and report you're willing to put up with, mitigated by how long a barrel you can comfortably wield.

If you have a caliber you really like, picture what sort of rifle you want to shoot it from, and that will help you decide the caliber.
I'd be a lot happier with a 7mm-08 than a 7mm RUM if I was building a lightweight mountain rifle with 20" bbl. But I wouldn't think of chambering a 30" rig with that same cartridge.

Bill Smith
I've always had a fancy for the 264 Win mag. If there was ever a caliber that was over bore this is it!
This caliber is next to useless here in Virginia. Hell, it's still accelerating when it hits something. {Yes, I know the bullet begins to slow down as soon as it leaves the barrel} For some reason that itty bitty bullet ripping along at 3500 to 3800 fps and the barrel lasting only thru the load testing process, seem to balance.
If I still lived out West, Mom's family in Caddo County Okla. and Dad's bunch in Beaver, I could sit on top of a gypsum hill and bust jacks over in the next county with that caliber.
Jim

Acording to Rocky Rabb, a noted writer and handloader,overbore is:
when case capacity (grns. water) is greater than bore diameter squared times 1000.
Regards, Curley

[ 07-06-2003, 16:29: Message edited by: curley ]

arkypete: yeah, you come from open country for sure. Beaver.....man that's way out there. that 264 would work well. i hunt around Stigler. if i ever saw a deer more than 40 yards away, i'd think i was in the wrong place..... . a 35 Rem is better suited for me..

all: thanks for the help. i'm a lot clearer on the matter now.

[ 07-07-2003, 08:29: Message edited by: bill smith ]

Over bore capacity has nothing to do with barrel length. Most certainly, a 25/06 can use more than 26" of barrel length! I would not be surprised if the "optimal barrel length" was closer to 26 feet than 26".

A commonly accepted definition of "over bore capacity" is that a maximum load of the slowest available powder does not fill the case. FWIW, Dutch.

I've always been under the impression that the term "overbore" related to velocity versus powder charge. For example, one cartridge may use 53 grains of powder and achieve 2850 fps with perhaps a 150 gr. bullet and another cartridge (in the same caliber) uses 61 grs. of powder but only gets about 3000 fps. with the same weight bullet. The powder charge is about 15% more (in this example) and the velocity is about 5% more. In other words, the law of diminishing returns. Bear in Fairbanks

[ 07-07-2003, 20:48: Message edited by: Bear in Fairbanks ]