"Over Bore"??


Active Member
Aug 13, 2001
Logan, UT
Okay, I have watched and read a ton of posts here over the past few months, and occasionally people have mentioned that a particular cartridge is "over bore capacity", or "over bore", etc. What exactly does this mean? Which cartridges fit into this category? Which cartridges are "at bore capacity" (I assume there is such a thing)? Thanks for the info.

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
It's hard to determine the case efficiency of the three 30s you mentioned.
The illistration I used concerning the large cases was just to show the large difference in Powder capacity cases and how inefficient one can be to the other using the SAME case.

The 308 Norma and 300 Win mag are both very good and very close.
The 300 ultra and 300 Tomahawk are both overbore capacity cases. They are however very good cases for the LR hunter so the big efficiency situation is not THAT important to us, as I explaned.

It would be hard to say which case is the BEST in any caliber you mentioned.

The 284 case is versatile, the longstanding 30/06 is very good, the 270 is very good. I could go on and on but, you will know what efficient is when you start to compare several cartriges in the same bullet diameter. When you use less powder to achieve the same velocity a large case will give you, that smaller case is many times more efficient then the larger case.

Most people think just because you can put more powder in a case you will get more velocity automatically. That's simply NOT true in every case.

Darryl Cassel

[ 01-14-2002: Message edited by: Darryl Cassel ]

Like Darryl said a cartridge being overbore is easiest to see in the very extreme cases.
A 50BMG is such a case, w/a case capacity of 290+ gr of H20 and average load being 220-240gr of powder. SSK industries now manufactures rifles chambered in 50Peacekeeper,
A BMG flanked by 2 Peackeeper rounds. A 460WBY case necked up to .510 cal. With a shorter barrel and half the powder they can achieve over 80% of the BMGs velocity with the same bullets.

A little more scaled down is my 270-300Ultra, It has a capacity of about 112gr H2O, vs a 270win w/a cap of 67gr H20. My ultra in a 30" barrel uses 103gr powder to push a bullet 3750fps while my 270win in a 28" barrel pushed the same bullet 3200fps using only 58gr of powder. The ultra takes 178% of the powder of the 270win (that's a 78%increase) to push the bullet 118% of the velocity of 270win( that's an increase of only 18%). It took 45gr extra powder to increase 550fps. That is very overbore, and very inefficient.

The most efficient cases are the smallest, a 300whisper, for example, can push a 240gr bullet around 12-1300 fps using 13-15gr of powder where a 300 Ulra pushes the same bullet 3000fps w/close to 100gr powder. The whisper has 43% of the ultras velocity while using 15% of the ultras powder. That's alot of speed for not alot of powder.
Thanks, Darryl.

Some follow-up questions: It sounds like the 6.5-284 is at or close to "optimum bore capacity", if there is such a hypothetical thing. What cartridges in the following calibers push that envelope of optimum powder-to-velocity ratio?

.284 cal
.308 cal
.338 cal

Additionally, you mentioned the .308 Norma as an efficient cartridge. How does the .308 Norma compare to the .300 WSM and the .300 Ultra cartridges for efficiency/bore capacity?

Thanks again!
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