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If three cartridges have identical case capacities and are loaded to identical pressures using the same powder/bullet/primer in identical rifles, wouldn't they have identical velocities?

For example: the 6mm-06 (water grain capacity 92gr), the 6mm/284 (water grain capacity 90gr), the 240 Wby Mag (water grain capacity 90gr) if chambered in identical 26" barrels and loaded to identical pressures with the same powder, primer and bullet. Isn't it reasonable to expect identical velocities?


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Yes.

My experience, in working with a great number of wildcats (usually no data), is that what you say is basically true. We would have to assume that the barrel(s) are absolutely identical, as well.

I'm the first to answer your post, so I will probably get run over by all kinds of scientific-types, claiming that one theory or another shows to the contrary. Take that for what it is worth.

Friend, you are entering the arena of Cowboy Ballistics and your premise works for me.

As an extension of your thought; does case shape shape have any effect on velocity and/or accuracy? My experience has been that it does not. I can hear the WSM and the PPC folks getting ready to do battle.

In my opinion, capacity equals velocity. Also, accuracy is a function of the 4 Bs; great barrel, superb bullets, excellent brass and perfect bedding. Add a C, for concentricity (case neck, bullet and chamber) and you have accuracy.

Steve


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If three cartridges have identical case capacities and are loaded to identical pressures using the same powder/bullet/primer in identical rifles, wouldn't they have identical velocities?

For example: the 6mm-06 (water grain capacity 92gr), the 6mm/284 (water grain capacity 90gr), the 240 Wby Mag (water grain capacity 90gr) if chambered in identical 26" barrels and loaded to identical pressures with the same powder, primer and bullet. Isn't it reasonable to expect identical velocities?


That assumption certainly felt reasonable to me, but I wondered what the differences would be � not just the similarities � so I ran the numbers on the computer.



� 6mm cartridge with 90 grains capacity, 95-grain Nosler Partition, Hodgdon H-870, and Pmax 60,000 lb/sq in.:

80.4 grains � 3,544 ft/sec



� 6mm cartridge with 92 grains capacity, 95-grain Nosler Partition, Hodgdon H-870, and Pmax 60,000 lb/sq in.:

81.5 grains � 3,545 ft/sec



Close enough?


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In my opinion, capacity equals velocity.


Completely agree. There's no such thing as a free lunch, nor a magical case shape to help. You might get a more efficient powder burn from one than the other and therefore better long range accuracy, but not likely more velocity. I've been playing with 338/300 WSM's for a while. The case capacity is generally about 5g, �1g, difference in water weight less than 338 Win Mag. While some have equalled the Win Mag in velocity, it had to do with long throats that relieved pressure in the WSM, which have very short throats. I suppose if the Win Mag was also long throated, it would again exceed the WSM.

Some years back I had a custom 338 Win Mag. In order to be able to carry two rifles for a pack-in hunt, and save weight in the ammo, I had a factory rifle rechambered with the same reamer as the custom. Figured that would solve most arguments with an outfitter about weight by having only one set of ammunition. To my surprise at the time, the factory rifle required one more grain of powder to equal the velocity and accuracy of the custom. When I slugged and measured both barrels I had the answer. Don't recall exact numbers now, but the custom was .0004" tighter than the other. It could have been .0007" difference, and there was the difference in velocity.


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Here's something from one of my recent articles ("A Family of Cartridges," The Accurate Rifle, August, 2003) re the idea that "capacity equals velocity" (actually, powder charge equals velocity).



I'm sorry, friends, but the word-processor of this forum compresses an otherwise easily readable table. I've tried to remedy this problem with dot leaders (...) to separate subcolumns and em dashes (�) to separate primary columns � HTH:

Quote
Blind reliance on the all-too-well established bar-stool or barber-shop (BS) ballistics makes it hard for some shooters to understand or accept the effects of certain proven interior-ballistics principles that make these cartridges work as I've designed them to work. The toughest knot for some to swallow is the fact that by (a) making the case of a new cartridge a good bit more capacious than that of an older cartridge, then (b) loading it with a larger charge but (c) not to its maximum safe pressures, (d) the same bullet can be propelled faster but without peak chamber pressures as high as those in the smaller cartridge at its lower velocity. This phenomenon has been a solid part of interior ballistics as long as there've been cartridges and rifles. The loading manuals are full of examples.



Let's look at the loads for two cartridges, chosen pretty much at random from the 50th Anniversary edition of the Sierra rifle manual � the obviously smaller .300 Savage and the obviously larger .30-06, both loaded with the Sierra 165-grain hollow-point boat-tail bullets, using the same seven powders. As the table shows, you can load the .30-06 somewhat lighter than it can stand, with slightly more powder than the top high-pressure loads in the .300 Savage, and gain a respectable amount of velocity at milder pressures.



Compare the powder charges and the muzzle velocities in italics with those to the right and the left of them. A few more grains of powder in the larger .30-06 case � but not enough more powder to raise the .30-06's peak pressures to their maximum � propel the bullets faster than the smaller charges in the Savage at maximum peak pressures for the smaller cartridge. The italicized velocities are not only higher than the .300 Savage could safely produce � their pressures are lower than the maximum for the .30-06, which means they're easier on the barrel than top-velocity loads in either of the two cartridges would be.



Similarly, all the Howell cartridges can be loaded to hotter velocities and higher peak pressures than I designed them for. But that's for the individual handloaders of these cartridges to decide for themselves � whether a little more velocity is worth more to them than longer barrel life. Longer accuracy life for the barrel is integral to the design philosophy behind these cartridges. It isn't the only purpose behind these designs, of course, but it's important to many of us.



.300 Savage, 165-gr Sierra HPBT � � � � � � .30-06, 165-gr Sierra HPBT

powder ... charge ... velocity (maximum) � � charge ... velocity � charge ... velocity (maximum)

IMR-3031 ... 38.0 gr ... 2,500 ft/sec � � 44.2 gr ... 2,600 ft/sec � 46.1 gr ... 2,700 ft/sec

W-748 ...... 40.6 gr ... 2,400 ft/sec �� 47.0 gr ... 2,600 ft/sec � 49.3 gr ... 2,700 ft/sec

BL-C(2) ..... 40.5 gr ... 2,400 ft/sec � � 45.7 gr ... 2,600 ft/sec � 47.5 gr ... 2,700 ft/sec

IMR-4895 ... 40.4 gr ... 2,500 ft/sec � � 45.7 gr ... 2,600 ft/sec � 47.8 gr ... 2,700 ft/sec

IMR-4064 ... 41.1 gr ... 2,500 ft/sec � � 47.9 gr ... 2,700 ft/sec � 49.6 gr ... 2,800 ft/sec

IMR-4320 ... 41.1 gr ... 2,500 ft/sec � � 48.9 gr ... 2,700 ft/sec � 50.9 gr ... 2,800 ft/sec

H-380 ........ 45.5 gr ... 2,400 ft/sec � � 49.2 gr ... 2,700 ft/sec � 51.1 gr ... 2,800 ft/sec



Some eagle-eyed soul is sure to notice in the Sierra manual that the test rifles had a 22-inch barrel on the .300 Savage and a 26-inch barrel on the .30-06. All right, duly noted � but that difference in barrel lengths is not enough to account for all the velocity gains in the submaximum .30-06 loads (as much as 300 ft/sec gain in velocity with H-380).
I hope this clarifies, a bit, the relationships that link case capacity, powder charge, velocity, and maximum pressure.


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I humbly agree with all of the above, and while on the subject.....Ken, how's the book coming? I need something to tide me over for recovery in a few weeks, you know, something light... [Linked Image]

I've read some of McPherson's work regarding hemispherical case shoulders on his SMC^2(I think) cartridges, and it is the only work I am aware of that is directed at improving efficiency via case shape. He seems to have some success at this, but for the most part a case is just a gasket, shape has no influence. Wonder where I read that..... <img src="/ubbthreads/images/graemlins/wink.gif" alt="" />


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I humbly agree with all of the above, and while on the subject.....Ken, how's the book coming? I need something to tide me over for recovery in a few weeks, you know, something light... [Linked Image]

Hey Digital

I've read some of McPherson's work regarding hemispherical case shoulders on his SMC^2(I think) cartridges, and it is the only work I am aware of that is directed at improving efficiency via case shape. He seems to have some success at this, but for the most part a case is just a gasket, shape has no influence. Wonder where I read that..... <img src="/ubbthreads/images/graemlins/wink.gif" alt="" />


Hey DigitalDan,

Isn't that true? The case is nothing but a gasket; the weak link of the entire system and the only thing standing between your face and a great deal of very hot gas.

I've often referred to the case as a gasket in articles and I believe Ken has, as well. Too few folks think of the case in this way but darn it, it's true.

Steve


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There have been magical claims about case shape since the invention of the darn things, and the claims just got worse when smokeless arrived.

I came up with the 4-to-1 capacity/velocity ratio (potential velocity increases or decreases at about 1/4 the rate of case capacity) a few years ago by crunching the numbers from all the manuals on hand. Comes out about right no matter the case shape, even in these days of short, fat magnums.

If, for instance, you have a .300 WSM on one hand and a .300 H&H on the other, you'll still get very similar velocities in similar barrel lengths at similar pressures. That's because case capacity is pretty close to the same in both. Same thing goes for 7mm SAUM and .280 Ackley, or .257 Roberts and .25 WSSm, etc. etc.

There might be some difference if you stretched the case out to a straight-sided job several inches long, or shortened it until it looked like a can of Copehagen with a neck sticking out the top..

Hmm. No, I ain't going there. It's hard enough to get the short-fats to feed.

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In smaller cases, with pronounced necks, case shape has little effect. I believe that is because the neck prevents much of the powder from moving into the barrel without being properly ignited. Long cases (over 2.5"), with minimal or no necks are a different story.

When the bore size goes up (starting at perhaps 35 cal), you can properly start to evaluate case shape. Take a case shape like the 9.3x74R, and compare it with something like the 376Steyer, or a WSM case. Or a 458Win with a Rigby based case.

In smaller calibers and shorter cases, no on has found a measurable difference without a battery of bench rest rifles that shoot averages in the .1" category. Not so in the larger bores. JMO, Dutch.


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DZ, to the best of my knowledge and belief it is the gospel. I had tonge in cheek a bit with the phrase, the description(gasket) is one I picked up in a really huge blue book about "Designing and Forming Custom Cartridges", by a gent named Ken Howell.

It is I think, a word worth a thousand pictures in this regard.


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Gents,
On the flip side which of the current modern cartridges seem underloaded when factory achieved velocity is compared to case capacity? First two that pop into my head are the 8mm Remington Magnum and possibly the 416 Remington Magnum.
Cheers...
Con


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