Case efficiency: fact or fiction?

Short and fat has generally been the trend for new cartridges. Yes, it seems to be a real thing.

Laphroaig said:

Short and fat has generally been the trend for new cartridges. Yes, it seems to be a real thing.

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I resemble that description.

seems that it’s mainly about case capacity but there also seems to be an uptick in performance when there is a heavy bottleneck. Just my simple observation but it seems as if maybe that sudden bottleneck is helping to build pressure quickly giving the bullet a better push for the first foot or so before everything normalizes. I’m no internal ballistics expert, but I do understand chokepoints and pressure.

Look this is physics. You can build equal or more pressure in a smaller space with less powder than you can in a larger space with more powder. If you compare .308 with 30-06 with the same bullet and the same powder you will find that the .308 will generate the same max pressure with a lighter charge than the 30-06. The only difference is the case length or volumn. It takes more powder to build the same pressure in a larger space.

Was that using the same propellant? If not this efficiency statment may not be true.

Fact (but I don't believe comparing cartridges pertains to "case efficiency"). Think of case capacity as similar to an internal combustion engine's combustion chamber. Size and shape matters...

Skeptic13 said:

Look this is physics. You can build equal or more pressure in a smaller space with less powder than you can in a larger space with more powder. If you compare .308 with 30-06 with the same bullet and the same powder you will find that the .308 will generate the same max pressure with a lighter charge than the 30-06. The only difference is the case length or volumn. It takes more powder to build the same pressure in a larger space.

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Case volumes can limit the amount of a powder, especially slower burning powders. Smaller case volume with certain powders will not allow the pressure to peak as desired.

Edit: I should have said you may not get a desired velocity. Say as in IMR4350. You will run out of velocity gain in 308 before you will out of 30.06 for the same pressure.

jski said:

Yesterday I received this while discussing the merits of the 25-06 Remington:

With factory ammo I don’t know what’s inside the case: which powder and how much of it? So I’m pondering whether the oft sited “case efficiency” is real? 20% heavier bullet and getting the same velocity with less powder. Hmmm?

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Huge overbore cartridges are not about efficiency. They are about maximum velosity for a given bullet size. If efficiency mattered most why would one choose 25-06 over 257 Robert's. That extra speed is about flat shooting at long range, and the cost is just accepted, just like a rebuild of a 10,000 hp funny car ever 1/4 of a mile.

6.5 PRC max pressure 65K PSI, 25-06 max pressure 63K PSI. "Might" have something to do with the difference for factory ammo,...

I'm sure on a scientific level there's some validity to it.

But it's not something I consider. I love the old .30-06, but I know there are dozens of cartridges that will do the same thing more efficiently. But I don't care.

swg1 said:

6.5 PRC max pressure 65K PSI, 25-06 max pressure 63K PSI. "Might" have something to do with the difference for factory ammo,...

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We also have the difference in bore diameter.

jski said:

Yesterday I received this while discussing the merits of the 25-06 Remington:

With factory ammo I don’t know what’s inside the case: which powder and how much of it? So I’m pondering whether the oft sited “case efficiency” is real? 20% heavier bullet and getting the same velocity with less powder. Hmmm?

Click to expand...

AJC1 said:

Huge overbore cartridges are not about efficiency. They are about maximum velosity for a given bullet size. If efficiency mattered most why would one choose 25-06 over 257 Robert's. That extra speed is about flat shooting at long range, and the cost is just accepted, just like a rebuild of a 10,000 hp funny car ever 1/4 of a mile.

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swg1 said:

6.5 PRC max pressure 65K PSI, 25-06 max pressure 63K PSI. "Might" have something to do with the difference for factory ammo,...

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Bfh_auto said:

We also have the difference in bore diameter.

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Apples to apples, .250ai vs .25-06

horsey300 said:

Apples to apples, .250ai vs .25-06

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That would be closer to a fair comparison.
I would want to see the same pressure, case capacity, bullet diameter, and powder charge.
Physics says the sharp bottle neck should spike pressure at the neck. How much is way beyond me.

Bfh_auto said:

That would be closer to a fair comparison.
I would want to see the same pressure, case capacity, bullet diameter, and powder charge.
Physics says the sharp bottle neck should spike pressure at the neck. How much is way beyond me.

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We could also compare (without starting a war for the love of pete!) 6.5c vs .260, when talking efficiency, I also factor in downrange performance, I can get to xxxx yds with how little powder.

Bfh_auto said:

We also have the difference in bore diameter.

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Yup, for an equal bullet weight, it is easier to push the larger diameter projectile, or in other words, sectional density matters. Keep in mind, there was a bit of sarcasm in my post.

Yup, for an equal bullet weight, it is easier to push the larger diameter projectile, or in other words, sectional density matters. Keep in mind, there was a bit of sarcasm in my post.

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Yea, I guess it depends on how you define efficiency. A 338 Federal will shoot the same bullet weights as 308 faster at the muzzle with similar powder charges. But in the same bullet weight the 308 will always catch up in speed at some point down range due to BC, and will always penetrate deeper due to SD. And for the same reasons 7-08 will beat 308.

Of course you reach a point where you go down in caliber to where the same, or even similar bullet weights aren't practical.

jmr40 said:

Yea, I guess it depends on how you define efficiency. A 338 Federal will shoot the same bullet weights as 308 faster at the muzzle with similar powder charges. But in the same bullet weight the 308 will always catch up in speed at some point down range due to BC, and will always penetrate deeper due to SD. And for the same reasons 7-08 will beat 308.

Of course you reach a point where you go down in caliber to where the same, or even similar bullet weights aren't practical.

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Correct.

FROGO207 said:

Was that using the same propellant? If not this efficiency statment may not be true.

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I think what people miss the most these days is operating pressure. Your not doing an apples to apples comparison of case to case if you are comparing one that has thousands of PSI higher operating pressures.

Why the old grains of water capacity used to be used quite a bit.

“Efficiency” really needs some parameters to be defined. “A bulldozer is more efficient at moving dirt.”, could be an accurate statement as well as, “A shovel and wheelbarrow is more efficient at moving dirt.” and even “A river is more efficient at moving dirt.” Just depends on how I choose to quantify efficiency.

While they are not an apples to apples comparison due to different capacities and often a slower burning powder in the larger capacity case, efficiency is a real thing as seen by comparing .308 and 30-06 using the same bullet in each. With for an example both shooting the same 150 gr bullet, the 308 is usually only 150-200fps slower yet uses about 20 grs less powder. In this case, the extra 20 grs of powder only nets a relatively small increase in velocity for such a large increase of powder charge that seems to be mostly just a recoil penalty since the performance increase isnt that great.
But efficiency is not everything as there are things the 06 can do that the 308 cant, mainly shoot 200+gr bullet.

Stevenvalleyky said:

But efficiency is not everything as there are things the 06 can do that the 308 cant, mainly shoot 200+gr bullet.

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Twist rate has a lot to do with that. Typical 1:10 for -06 and 1:12 for .308. I never understood that

Laphroaig said:

Short and fat has generally been the trend for new cartridges. Yes, it seems to be a real thing.

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Why the Remington BR cases were so often sought after to make wildcats. The 6 and 7mm BR cases are awesome for efficiency

That might have been the case with the original rifles but most of the 308s Ive seen in the last few years have come with 1:10. I believe but am not certain that most of the cheapy rifles like the axis and Americans come with 1:10 in both 30-06 and 308.
Im assuming the reasoning for that would be with the 308s smaller case capacity, the 308 would generally be shooting lighter bullets and thus the slower twist.