What is the most overbore cartridge YOU HAVE USED?

[MTSLW]

22x55 Swede Improved ~52gr RL26
88eld 3600+

 

[Calvin45]

A Kirby Allen special. 300 Raptor 245's 3400fps 127gr powder

In the more distant future I plan to do a 30-33xc which is about the same thing (acknowledging the raptor came first!)

Probably build off a savage 112 magnum target action (or just leave it alone and shoot the .338 lapua haha)

 

[Calvin45]

In my opinion, not enough attention is paid to Expansion Ratio.

I think a better, more equal, way to compare external ballistics is:

Powder Load / Barrel Volume
where barrel volume = bore squared x barrel length x Pi/4

This gives results most shooters have not seen.

As an example, comparing the ratios of 30-06 charge/barrel vol with 30-378
charge/barrel vol, if you use a 22-in barrel for a 30-06, the same expansion ratio
in a 30-378 yields about a 44-in barrel requirement.

Nobody builds a 30-378 that way, so in my opinion, the 30-378 never gets a fair comparison.

This is only one example.

***EDIT***
I think this is Internal ballistics, not external ballistics.

Makes sense though. I've often wondered where the appeal is on a 26 inch 10 twist 30-378 or .300 warbird or similar things…burn another 25-50 grains of powder to beat the normal .30 cal magnums by a measly 200-300feet per second. But I hear in pipes 30 inches and longer the drag racers really start to pull ahead, or more so increasing barrel length doesn't lead to the same kind of increased velocity with a .30-06 as it will with a .30-378 to use your comparison.

Total volume for the powder to burn and gasses to expand and work on the bullet as defined by case capacity and barrel internal volume makes a lot of sense compared to only considering case capacity. Case capacity tells you how much powder you can ignite but not how much work it's gonna be able to do.

By that reasoning a .30-06 is appropriate in a 24 inch barrel and definitely "overbore" in an 8 inch barrel. That makes sense to me.

 

[Calvin45]

The Middlestead is a Bad Egg, I still have all of my stuff to build another one

Is a bad egg a good thing or a bad thing?

 

[6.5x300 WBY]

Makes sense though. I've often wondered where the appeal is on a 26 inch 10 twist 30-378 or .300 warbird or similar things…burn another 25-50 grains of powder to beat the normal .30 cal magnums by a measly 200-300feet per second. But I hear in pipes 30 inches and longer the drag racers really start to pull ahead, or more so increasing barrel length doesn't lead to the same kind of increased velocity with a .30-06 as it will with a .30-378 to use your comparison.

Total volume for the powder to burn and gasses to expand and work on the bullet as defined by case capacity and barrel internal volume makes a lot of sense compared to only considering case capacity. Case capacity tells you how much powder you can ignite but not how much work it's gonna be able to do.

By that reasoning a .30-06 is appropriate in a 24 inch barrel and definitely "overbore" in an 8 inch barrel. That makes sense to me.

I'll happily burn 25 to 40 gr more powder for 200-300 fps increase. Very very very happy to do it. Every day all day long

 

[Calvin45]

I'll happily burn 25 to 40 gr more powder for 200-300 fps increase. Very very very happy to do it. Every day all day long

Are you happy to run a longer barrel to get full potential? I'm happy to burn more powder for performance too, I just don't get why a person would go to all the trouble and inefficiency and then put a shorter barrel on it to neuter its performance.

 

[6.5x300 WBY]

Are you happy to run a longer barrel to get full potential? I'm happy to burn more powder for performance too, I just don't get why a person would go to all the trouble and inefficiency and then put a shorter barrel on it to neuter its performance.

If I'm shooting a factory rifle aka weatherby in 30-378 with 26" tube vs custom rifle I'll shoot whatever it comes with till it's gone for rebarreling. Then it's gonna be longer 28" minimum if not 30"

 

[wasskeet]

It is potentially what I am running now. A 6 PRC, loading H1000 with 109 hybrids at present and getting pretty good speed and accuracy. Tried Retumbo - it no likey that at all.

 

[hemiford]

Makes sense though. I've often wondered where the appeal is on a 26 inch 10 twist 30-378 or .300 warbird or similar things…burn another 25-50 grains of powder to beat the normal .30 cal magnums by a measly 200-300feet per second. But I hear in pipes 30 inches and longer the drag racers really start to pull ahead, or more so increasing barrel length doesn't lead to the same kind of increased velocity with a .30-06 as it will with a .30-378 to use your comparison.

Total volume for the powder to burn and gasses to expand and work on the bullet as defined by case capacity and barrel internal volume makes a lot of sense compared to only considering case capacity. Case capacity tells you how much powder you can ignite but not how much work it's gonna be able to do.

By that reasoning a .30-06 is appropriate in a 24 inch barrel and definitely "overbore" in an 8 inch barrel. That makes sense to me.

Another illustration of energy:

If cartridge B has twice the powder as cartridge A, it will impart twice the energy to the bullet,
not twice the velocity.

If cartridges A and B both shoot the exact same bullet (same mass),
than the bullet shot from B will have 1.41421 times the muzzle velocity of A (sq root of 2).
So if muzzle velocity of A is 3000 ft/sec, B will be 4243 ft/sec.
B would need twice the barrel length as A to do that (same expansion ratio).

Also I'm assuming the same powder in each, or at least powder with the same energy content.

Now it may be true that ultra long barrels have increased bullet drag, so you would
lose some energy and velocity from that effect.

 

[Lefty7mmstw]

Another illustration of energy:

If cartridge B has twice the powder as cartridge A, it will impart twice the energy to the bullet,
not twice the velocity.

If cartridges A and B both shoot the exact same bullet (same mass),
than the bullet shot from B will have 1.41421 times the muzzle velocity of A (sq root of 2).
So if muzzle velocity of A is 3000 ft/sec, B will be 4243 ft/sec.
B would need twice the barrel length as A to do that (same expansion ratio).

Also I'm assuming the same powder in each, or at least powder with the same energy content.

Now it may be true that ultra long barrels have increased bullet drag, so you would
lose some energy and velocity from that effect.

The problem you start to run into on the really hot ones is the velocity of the gas, isn't it? I'm under the understanding the gas expansion is around 5,000 fps, so no matter what you do with standard nitro based powders you can't really get above that speed.
When I went for 4,000 fps in a 7rum, I was burning 109gr. or so aa8700 behind a 120 Sierra and hitting just a few fps below the magical 4,000 fps mark. That's nearly 1 gr. powder for every grain of bullet.

 

[HRM Johnny]

Don't know if this goes here or on reloading forum, but figured I'd ask…

What is the most overbore cartridge you ACTUALLY have experience with? The craziest hot rod, the biggest case to bore ratio, the most ridiculous magnum, this worst barrel burner….

What have you learned from it? Would you do it again?

I know everyone on these threads makes jokes about a .17-50bmg or references the .22-378 eargesplittenloudenboomer experiment by ackley…I don't care about those. I don't know of a single person who has ever done such a thing in any serious effort. I'm trying to avoid that and learn about people's experiences and learning from actual field rifles in very overbore factory chamberings or more so truly impractical but still useful wildcats. I'm entertaining the thought of one myself.

@ButterBean @Fiftydriver @MagnumManiac i know you folks off the top of my head have played with some big cases pushing small bullets….

Great thread. Thanks for starting this.

 

[Calvin45]

Another illustration of energy:

If cartridge B has twice the powder as cartridge A, it will impart twice the energy to the bullet,
not twice the velocity.

If cartridges A and B both shoot the exact same bullet (same mass),
than the bullet shot from B will have 1.41421 times the muzzle velocity of A (sq root of 2).
So if muzzle velocity of A is 3000 ft/sec, B will be 4243 ft/sec.
B would need twice the barrel length as A to do that (same expansion ratio).

Also I'm assuming the same powder in each, or at least powder with the same energy content.

Now it may be true that ultra long barrels have increased bullet drag, so you would
lose some energy and velocity from that effect.

In theory yes in reality no. Twice the powder will only impart twice the energy IF the system is PERFECTLY efficient. But (pesky thermodynamics and entropy) no system is ever perfectly efficient and the more "overbore" you get the more inefficient it becomes. A .30-378 may burn twice the powder of a .30-06, but at equal pressures and equal barrel lengths (heck even at longer ones) it sure ain't generating twice the energy,

You're absolutely right about speed tho, inverse square law, a bullet of equal weight moving twice as fast hits four times as hard, three times as fast hits nine times as hard, and so on.

This is a part of why 400 feet per second difference is actually A REALLY BIG performance change, even though when viewed from a percentage standpoint it may not seem that much different (2800 vs 3200 fps for example)

The other factor that makes these inefficient cartridges worth doing to some for purely hunting purposes has less to do with trajectory and much more to do with impact velocity and "magic numbers" - 1800 and 2600 fps impact velocity coming to mind. Below 1800 many bullets do not expand or upset reliably, poor erratic performance. At impacts of 2600 and up the smallbores (under 338 cal) hit notably different, the odds of a DRT poleaxe kill without a cns hit goes up dramatically (as per Nathan Foster's observations of thousands of animals Shot, i beleive he's right and my very limited experience is similar).

If starting from 0 fps as a threshold 3200 might not be that much faster than 2800. If starting from 1800 fps as a minimum threshold it's much faster, longer capable range by far….and if it's starting from 2600 as a desired minimum impact speed then 3200 is much much much faster indeed

 

[Calvin45]

The problem you start to run into on the really hot ones is the velocity of the gas, isn't it? I'm under the understanding the gas expansion is around 5,000 fps, so no matter what you do with standard nitro based powders you can't really get above that speed.
When I went for 4,000 fps in a 7rum, I was burning 109gr. or so aa8700 behind a 120 Sierra and hitting just a few fps below the magical 4,000 fps mark. That's nearly 1 gr. powder for every grain of bullet.

The expansion velocity is a good bit faster than that, I know the .22-284 has exceeded 5000, and the 300 win mag with those accelerator type sabots has exceeded 5200 with 22 cal sabots bullets.
The 30-378 was originally meant to test shrapnel armor and used a weird 30 grain (I think) tin/aluminum type alloy at 6000+ feet per second.

I honestly think with the super overbore cartridges you just need to accept ridiculously looooong barrels to realize the increases possible.

For what it's worth, I've exceeded 4000 fps in my .300 win with 120 Barnes tac tx and in my .257 weatherby with 75 grain hammers.

 

[MagnumManiac]

I answered what was the worst for me, but if we are all adding other rounds that are also overbore, I will add a few.
25-06, 25-300WSM, 6.5-300WSM, 257 Weatherby & 270 Weatherby. I don't feel the 300 & 340 Weatherby are truly overbore, but my 300RUM probably is…
I am also a believer in Expansion Ratio and shoulder angles making huge differences to outcomes on cartridge design.

Cheers.