Bore to charge ratio? Optimum powder results for accuracy...

[Socrates]

Hi

I'm trying to remember how the relationship between the 30-06 and the .308 was described to me. It was something like with a given powder amount, to a certain bore size and projectile, you get an optimum situation that promotes accuracy.

Key in these factors are a nearly full case, the powder operating in the pressure range that is ideal for it, and the shooter, and a bullet that is optimum for caliber for accuracy.

The guy used the terms,
"overbore"
"underbore"
"ideal"
etc.

Anyone remember the exact buzz words to describe this situation, and design parameters?

[Jimro]

What he was talking about is "efficiency".

A 308 will use less powder than a 30-06 to launch a 147 grain bullet at the same velocity, this means the 308 is more efficient with that bullet mass. You can divide the the velocity achieved by the charge weight to get a fps/gr rating.

As long as the bullet weight is the same you can compare cartridge efficiency this way.

Now as bullet mass increases the 308 runs out of "oomph" to push the heavier pills, which is where the increased case capacity of the 30-06 allows it to be a better performer. If you compare the 30-06 to the 300 Win Mag you see the same relationship, the 30-06 is more "efficient" but it can't push the bullets as fast as mass increases.

Now the term "overbore" and "underbore" are usually thrown about by fanboys who like to feel good about a particular cartridge of which they are fond. A 30-378 Weatherby is "overbore" but it is also a dang good round for 1,000 yard shooting.

So take a look at what you intend to do with a rifle/round combination, and then decide if it meets your needs. If you actually want to find the equations that compare case capacity to bore diameter you can google them, but it won't affect the way your rifle shoots.

Jimro

[Sidetracked]

"Bore Capacity"

P.O. Ackley covers it pretty well in Vol. 1 of his Handbook for Shooters and Reloaders.

I would try to summarize, but a lot of information is lost that may, or may not be important to your interpretation.

Get the book. It's a good reference. ;)

[apr1775]

Imagine a very large case necked down to a very small bullet. When the round is fired and the bullet leaves the barrel, there is still a lot of powder burning. There's a lot of left over energy in the form of muzzle blast and flash. Those kind of cartridges can produce excellent long range ballistics because bore pressure is maintained high during the bullets entire travel down the barrel, especially with a longer barrel, but a lot of energy left over after the bullet leaves the muzzle.

I can compare my stock toyota corolla to my hot rod camaro. The corolla is more efficient and that's why I drive it most of the time, but the camaro is much faster, it just takes a lot of gas to make it go fast.

[Art Eatman]

I guess the term "overbore" has come into usage in the last fifty years or so, with all the R&D that went on with the .308 and other such cartridges.

In comparing factory loadings of the .308 and the .30-'06, several things are important to remember. First off, the '06 loads were commonly limited to around 50,000 psi because of all the older rifles still in use. The .308 started out at 55,000 psi.

Since the '06 is slightly overbore, it performs better with slower powders (4064, H414) and longer barrels (24" to 26"). The .308 does quite well with 20" to 22" barrels.

The '06 can get higher velocities with the heavier bullets because they need not be seated so deeply that they intrude into the powder volume--yet the overall length limit is not disturbed.

For a handloader with a 26" barrelled '06, the advantage increases notably. For equal chamber pressures, the advantage is 200 to 300 feet per second at the muzzle.