the following data is from the Barnes manual:

case capacity:
7x57: 55.55 grs of water
7-08: 52.23 grs of water

the following data is from the Alliant manual:

load using 160 grain bullet:
7x57: 49 gr RL-19 @ 2665 fps and 45,500 psi
7-08: 48.5 gr RL-19 @ 2675 fps and 56,400 psi

my question: the load data is nearly identical, as is the performance data. the case capacity of the 7-08 is only 6% less than the 7x57, but the pressure is 24% greater. is the case capacity the only factor here??

[ 11-10-2003, 07:11: Message edited by: bill smith ]

The case capacity is only one part it. The chamber is the true pressure vessal, lenght of the leade, powder lot, bullet brand (Seirra, Hornady, ect) bore smoothness, and how many virgins you`ve sacrificed to the pressure gods that day all add to it.
Case capacity puts you in the ballpark but it needs to be fine tuned from there. I find it rare that two labs come to the same conclusion on what charge of a given powder with the same bullet is max in the same cartridge. I`m sure in there is a relationship that can be shown mathematicaly in a perfect world but, in the real world too many variables exist.

Ummm SHAPE!
The 308 case is more or less an Ackley chamber.
If you compare the 7x57 Ackley, you should find a big difference between the 7-08 vs 7x57AI.

Bolt thrust is reduced allowing higher pressures.

More powder, less bolt thrust, in more times than not, will produce higher velocities.

To much fun...

I have several Ken Waters articles on both. He maintains that his several 7x57's require one to two grains more powder with a given bullet weight to equal the 7-08's velocity with that bullet weight. A long military throat could be the culprit here with the 7x57...

Powder space is also a factor....

It is like an engine, air fuel ratio...

To much fun....

Mauserkid: Do you really believe that an Ackley improved chamber will
have less bolt thrust?. If so why does a case "stretch".
Good luck!

The less the case taper, the less the backthrust on the bolt. This is caused by such cases gripping the chamber walls better, and not trying to back out of the chamber at high pressures.

A couple of the worst cases in this regard are the .300 H&H and the .280 Ross. I think Ackley pretty well proved this experimentally, and one of the best designs for lowering backthrust against the bolt, and consequently giving very easy primary extraction, was Rocky Gibbs's line of "improved" cartridges using '06 brass. One could literally open the bolt on a Gibbs rifle with the little finger after firing.

Notice if you will that you are dealing with a 160 gr. bullet. In most commercial 7m/m-08s this bullet would have to be seated deeply into the powder room to fit into the magazines and throating. This reduces your initial available case volume (powder room).Instead of a 6% difference you might be looking at a 12% difference depending on how far out you are able to seat the bullet in the 7m/mx57. Use of the 175 gr pill would be even more demonstrative. I think all cartriges are nice, even the poorly designed such as the 7m/m-08 that has been adabted to an improperly lengthed rifle!!!!!

Note,,, Rocky Gibbs polished his chambers to a mirror like finish...

This is a good discussion....

just a question, but why would you need a 160 gr bullet in the 7mm08? the 139gr hornady is plenty. kind of in the same vane as the 130gr in the 270 as opposed to the 150gr what do you guys think????

Smallfry, how does my statement make me look bad?

quote:

---

Ummm SHAPE!
The 308 case is more or less an Ackley chamber.
If you compare the 7x57 Ackley, you should find a big difference between the 7-08 vs 7x57AI.

Bolt thrust is reduced allowing higher pressures.

More powder, less bolt thrust, in more times than not, will produce higher velocities

---

The case design of a 0-8, or Ackley allows the pressure to be more evenly distributed on the chamber walls than against the bolt itself. This is also supposed to keep you from seeing false pressure signs.


If I am not correct, please show me what is correct. Or better yet teach me what is correct

Thanks...

[QUOTE]Originally posted by d l:
[QB]"just a question, but why would you need a 160 gr bullet in the 7mm08?" Don you only need it for this discussion in that it initiated the thread. I have hunted in Colorado, when I lived there, with a lot less than a 7m/m-08 and a 139 gr. bullet. The acceptable norms of today may think me to have been a little imprudent. They may be right.

Ok Mauserkid... brief and basic...
Pascals Law states that presure added to a confined fluid at any point instantly appears equally at all other points, regardless of the shape. You can use this principal for cartridges. Inside area of the case at the base X PSI would give you one set of figures... Or outside would suffice. But where are you going with bolt thrust? Is it the locking lugs you are concerned about? You would have to understande that what you calculate for bolt thrust will not be the same figures your locking lugs "see".

Someone else on this thread mentioned the case gripping the chamber walls and having less bolt thrust... this is *somewhat* true. By *somewhat* I mean the figures would be nominal to the point that I would doubt I would see the results in a over the counter pressure tester(The Coef. of friction between a rough chamber and a "smooth" chamber is a small change, especially when you consider its brass in steel.). The problem with the case gripping the chamber wall is... that when the cartridge is full length resized, and then fired... the case will move forward, "grip the wall", then grow or stretch the case head back to seat on the bolt face. The pressure (same) will still be being applied to the base of the cartridge, regardless of weather it is contacting the bolt face (which it always will at some point in time). In respect to the case "gripping the chamber"... the time delay between ignition and contact of the bolt face will be longer and the total time in contact will be less, this is where your nominal results will be.


If I gave two shits, I would opt for a smooth chamber, keeping as much as the "scary" bolt thrust on the locking lugs as could... just where it is suppose to be, I would also concern myself more with sizing/forming my cases correctly.

Hope that helps a bit Mauserkid

[ 11-12-2003, 09:25: Message edited by: smallfry ]

Hmmm...

Good answer, but>>>>

The physics you state make absolute sense. I also believe that you are correct in your statement. What I do not believe is the fact you have left out the law of Mechanics. Unfortunately for me, I cant quote any super smart guy theory. What I do know is that we are allowed to load straighter walled cases to higher pressures, and believe your Physics answer does apply. It just does not answer the question of variables and moving parts.

Sorry for the not agreeing with you 100% stuff, I just cannot see how it does not explain higher pressures in straighter walled bottleneck cartridges of the world. And this is where the mechanics come to play. No wedge, now locking brass against the bolt. Also reduces brass flow, and my 6.5x55 BJAI don�t know any better.... I am not comparing weak actions with stronger actions in my variable. Lets just pretend all things are equal except the wall taper.

By the way, I neck size.

Again, I am willing to learn more if your willing to educate..

Please do not take offence to any of this. I see your point, and do understand the law of Pascal you quoted. But I feel there are other factors involved than just Physics here. I hope I am getting my point across correctly.
I also hope I am not making a fool of my self, and just plane missing somthing.

To much fun....

[ 11-12-2003, 10:31: Message edited by: Mauserkid ]

Cases that "grip" the chamber walls can reduce casehead thrust, but only by the amount of force that it takes to stretch the case, a relatively small amount.

I don't like to run stuff so close I have to count on something like that. And, yes, I shoot a couple of "Improved" cartridges.

Ask yourself how can the shape of the shoulder affect the bolt thrust a only that part of the case back of the "expansion line" moves ?.
Any case seems to stay forward (having been pushed there by the primer)
up to around 40 KPSI or so. That is why primeers protrude from lower pressure
cartridges when fired in a chamber with headspace. If the cartridge is higher pressure the case head will push back (having stretched) and "rivet " the primer.
I have been playing around with tape that indicated pressure, and it would seem
that once the case head moves back it hits the brech with the same force as a n
oiled case. (This needs more work to be positive).
Finally Ackley did fire an improved 30/30 without locking lugs without it,
blowing back. What he didn't do was fire a standard 30/30 it would have stayed forward also.

Good luck!

Ok guys, I understand your points. I still believe shape has a lot to do with this law of your physics. In fact there are a lot more things happening than just gas expanding creating pressure.
The chamber is a fixed vessel; it has a hole, and a lid (bolt). Lets add in a non elastic container.(brass casing).
If you have a container that has a lot of taper, and the chamber is shaped the same. The brass flows in the direction of force. It travels out the hole, and applies pressure to the bolt. This gives you an indication of Over Pressure. (Hard Bolt Lift) This will happen at lower pressures.

Now lets change the shape of your fixed vessel, and a container that is formed to match. (Straight Walled Bottleneck Case.) My belief is that the flow of brass is slowed down, or possible redirected. This allows for more pressure to be created before brass flows to hold pressure on the lid. (Bolt lift reduced)

If one could eliminate the thin walled container, I would have to assume that your law of physics would be 100% correct.

Chamber angles deflect the expanding pressures as well; this would and does assist in brass flow. By reducing brass flow, then you would be reducing false pressure signs.

I may not be reading your statements correctly, and misinterpreting them. The one thing no one has iterated is the fact that there are higher-pressure cartridges out there.
The one thing that they have in common is the fact that the shoulder diameter is almost the same diameter as their base. The 0-8 case is .473 at the base, and .454 at the shoulder. My 6.5x55, which is notorious for brass growth with an improved shoulder, this problem has completely stopped. In fact it shows signs of shrinking.

As indicated I would need some real expensive test equipment to prove all my theories.

This is a very educational forum.

Thanks�.

[ 11-12-2003, 19:35: Message edited by: Mauserkid ]

I'm still of the opinion the longer than average throat of most 7x57's is the reason for lower velocities...

Longer throat will be the difference, effectively making the 7x57 much larger. I don't know about the case shape idea or the physics involved, but a sloping case such as .375 H&h will definitely start to show extraction problems at a lower pressure than say a .338. I know Ackley could improve the case on a .30-30 Model 94 with the locking lugs removed, only the lever locking the action up, this is nothing new, from his old manual from the 50's.

Mauserkid; Brass "flows" forward (not backward) and thickens the neck.
The case stretches back and becomes thinner at the "pressure ring".
Other than that your statement makes sense.
Good luck1

And the answer is;all else being equal,CASE CAPACITY HANDS DOWN. The rest is smoke and mirrors. Roger

Gee Hot Core ,You make me feel like a retard. Vector anaysis in high scool? I didn't study that till I was a junior or maybe a senior in colledge. But than I don't spelll good either. roger

Hey Roger, Maybe it was after High School. It has been a long time ago, in either case, for me.

The thread peaked my interest and I got out P.O. Ackley's "Handbook(Vol 1) For Shooters & Reloaders" to re-read his Testing done with the M94 30-30 last night. Good reading which is a bit "dated" in some areas, but excellent overall.

Irv...

quote:

---

Brass "flows" forward (not backward) and thickens the neck.
The case stretches back and becomes thinner at the "pressure ring".

---

This could be true, or not�..

There are a couple of reasons that come to mind that makes it difficult to (see) why I say this.

First, the chamber dimensions are larger at the base, and brass flows (stretches) around.

Second, the firing pin moves the case forward during firing. This re-stretches the case.

What I can see is the fact that the Ackley case is shrinking in my rifle, this would back up my theory that brass will flow in both directions. It is just more or less in different shaped chambers.

Way cool...