Hello
The Overbore cartridge is one that is very inefficient as per bore diameter, amount of powder used and also bullet diameter.
Lets use the 378 Weatherby case for instance.
Lets neck the 378 case down to 338, 308, 7mm and the 6.5. The most overbore cartridge here would be the 6.5/378 Weatherby. The reason being, it would take 20 or more grains of powder in this BADLY overbore case to duplicate a 6.5/300 Weatherby ballistics, plus the barrel wear is much more rapid in the 6.5/378 case. The 6.5/300 Weatherby is also overbore but, not to the extreme the 6.5/378 would be.
The best bullet diameter to shoot in the 378 case in the above senerio would certainly be the 338.
Lets match up the 30/378 Weatherby to some of the 30 cal Imp cases (normal imp. mags) such as the 30 Goodling, the 308 Baer, the 300 Ackley, 300 DC Super IMP using the 300 Weatherby as the parent case.
You will find that you can duplicate velocities of the 30/378 with a lessor powder charge. That's because the bore to case volumn in the 30/378 is overbore capacity.
We have run tests on this years ago and the cronagraph don't lie.
I Improved an 8MM Mag case years ago and called it the "300 Super Imp DC" --That case on a standard magnum action will duplicate 30/378 Ballistics. I talked to Bruce Baer about that and he said the same thing about his 308 Baer case.
The difference between his case and mine is;
mine is .002" larger in diameter at the shoulder, I have a 40 degree shoulder (Ackley) on mine and the Baer has a 35 Degree and I used the 8 Mag as the parent case and Bruce used the 300 Weatherby as the parent case. The 8 mag is a bit longer but, not much.
The 264 Mag is another example of Overbore.
You can use the 6.5 Gibbs cartridge (30/06 or 270 as the parent case) and duplicate the velocities of the 264 mag with LESS powder and using the SAME weight bullet with the same barrel length. We have done the tests on this one too.
IF you want an efficient 30 cal cartridge and you want one that will use less powder and accomplish almost the same velocity as some of the bigger mags, check out the 308 Norma mag.
This was once a VERY popular cartridge at Williamsport. It could duplicate velocities of the 300 Weatherby using less powder to do so. The problem was the quality brass at the time. You can now get some good brass for that cartridge. Don't use the reported velocities by Weatherby because their rifles are "Freebored" too much and this is why they report the velocities they do. Most any rifle with a lot of freebore (can't get close enough to the rifle lands with the bullet) are NOT very accurate. The best way to use the Weatherby case is to eleminate the freebore and chamber so that the bullet will JUST touch the lands when you adjust your seater die to do so.
As for Longrange hunters, we try to drive the bullet at extreme speeds to utilize the downrange energy and maintain velocty.
Most of our cases are overbore but, we push the envelope a bit here and really don't care if the cartridge is a bit overbore.
We many times go to the extreme because there are no loading books available for some of our cartridges. It's trial and error.
Hope that helped
Darryl Cassel