I know it's probably been here before but I have to ask.

While pouring over a Speer reloading manual I have trying to decide what caliber to have my 1999 shorts chambered to I found the following concerning the 7mm-08 vs. the 7x57 Mauser.

Using the identical 175g Speer bullets, and only comparing the loads using identical powders, the 7mm-08 consistently get more muzzle velocity using less powder.

Now I understand that the standard operating pressure (SOP) of the 7x57 is less than the 7mm-08 (46000cup vs 52000cup) but isn't SOP a function of how much powder is used in the case?

Splain it to me. Please

Rob

Pressure is also related to "how" the powder burns. Longer, tapered cases create less pressure than shorter, sharp shouldered cases - Even if the they use a little more powder.

If you handload the 7-08 will always run a weak second to the 7x57.

DB Bill,

You said....

"If you handload the 7-08 will always run a weak second to the 7x57."

Why would you say that if the 7mm-08 gets more MV with less powder?

Rob

NO - identical peak pressures won't always show the same results. The case capacity has a lot to do with how fast a bullet will be at the muzzle. This explaination is kinda involved and the calibers might not be what you expect, but they make the math easier.

You have fired a 454 Casull (1.4" case) and a 458 Winchester (2.5" case) cartridge each with a 350 grain bullet that is now 1/2" out of the case. Assume all the powder is burned and converted to hot gases, and the pressure is now at the peak of 62,000 psi. For each caliber you have a roughly cylinder shaped volume behind the bullet, with a volume length of 2" for the 454, and 3" for the 458; with roughly equal cylinder diameters. These volumes are different sizes but start at the same peak pressure. When the bullet moves 2 inches farther down the bore, the pressure of the 454 Casull will be cut in half (31,000 psi) since the volume behind the bullet doubled (cylinder length went from 2" to 4"), while the pressure of the 458 Win will be at about three fifths (37,000 psi) as the larger volume of the 458 Win case means the volume behind it's bullet only went up by two thirds (cylinder length went from 3" to 5") instead of doubling like the 454. This is the difference of case capacity - you can fit more powder in a larger case resulting in a larger volume of high pressure gas, so the pressure won't drop as fast as the bullet goes down the bore. This means more "push" on the bullet, so it goes faster from the larger case, even though the peak pressure was the same as with the smaller case. It also demonstrates why there aren't huge velocity gains as cases go from small to large to huge - the pressure does fall faster in a cartridge with a small case, but not that much faster than a large case.

If you graph out the pressure behind the bullet versus the position of the bullet in the barrel, the velocity of the bullet will depend on the area under the line - so with a big case you can use less peak pressure and still get a bullet going as fast or faster than a small case at high pressures. This is the difference between the old British Nitro Express cartridges with huge case capacity and low pressures versus more modern smaller cartridges that run at much higher pressures.

Are these loads using identical powders, or is one slower than the other?
More powdwer = more speed, but if you load a small case to 90% load density and then try the exact same load in a bigger case, which ends up with only 70% load density, I wouldn't be surprised to see 50-75fps deffierence.
But load them both to 90% and see how they compare. Add to that the fact that they may not both respond optimally to the same burning rates.

If you are using a modern rifle, there is no reason to load the 7x57 to less pressure than the 7mm-08. But as always, each rifle is an entity unto itself, and all normal rules apply.

Case shape has nothing to do with how much pressure a particular cartridge produces!! Pressure is a matter of four factors: case capacity vs bore diameter; bullet sectional desnsity; amount of powder; and type of powder. Theoretically, a large case using a large amount of slower powder can actually fire a given weight bullet at higher velocities using less peak pressure that a smaller capacity case using faster powders can, even if the smaller case operates at higher pressures! The reason for this is although the peak pressure in the larger case is lower, pressure is maintained for a longer period of time, keeping the pressures higher OVERALL as the bullet is accelerating, so it's final velocity upon exiting the bore is higher. However, a longer barrel is required to maximize this effect.

The 7X57mm cartridge can be safely loaded to as high a peak pressure as the 7mm/08 can, (provided the rifle it is fired in is a modern, strong one like a Ruger 77, No.1, Rem. 700, Model 70 Win., etc). And if you load the 7X57 to the same pressure levels as the 7mm/08 using the SAME powder, the 7X57mm will produce a higher MV than the 7mm/08, because it will take MORE of that powder in the 7X57mm to get the same peak pressure, so there's a greater amount of gas in the 7X57 to continue pushing on the bullet after the 7mm/08 has pooped out!! But, it takes LESS powder in the 7mm/08 to get to the same peak pressure, because it has a smaller volume!!

[ 07-27-2003, 16:09: Message edited by: eldeguello ]

As I said, I'm a newbie to this stuff and I can see valid points in each of the above explanations. Again, the info I took from the manual showed identical bullets and indentical powders
What I gather is because of the case shape, the 7x57 needs a bit more powder to produce the same MV as the 7mm-08.

I expect that the maximum listed load of 52kcup for 7mm-08 fills the case to capacity, while the max listed load of 46Kcup for the 7x57 leaves room for more powder. Therefore the 7x57 can be loaded to higher pressure (in modern actions) than the manual states and getting it up to 52Kcup will provide a significant increase in MV.

Thanks men.

Rob

I do believe that bullet pull (neck thickness/length) and throat length can be significant factors as well.
Variations in brass manufacturers, etc., will produce variations in bullet speed.
There's no telling the specifics of the two chambers Speer used.
Bottom line, if you like the speeds of the 7mm-08, it may be the perfect choice for you.
If you're looking for which is faster....

I would liek to add that the reason the 7x57 is loaded so low buy the ammunition companys is due to the fact of the amount of surplus firearms around in 7x57 that would not be suitable for modern pressures. So the makers load to a lower pressure to avoid lawsuits, so for the 7x57 to shine, you really need to handload it for a particular rifle.

Isnt this a scaled down version of the 308 vs 30-06 debate? I believe that the general consensus there is that the 308 runs right with the 06 until large bullets are used is it not? Why then would the 7-08 not be running right along side the 7X57 which has LESS of a capacity advantage over the 7-08 then the 06 does over a 308??

The thing you need to look at in the Speer book is that the 08 is loaded to 52,000 CUP. They only push the 7X57 to 50,000 CUP. The 7X57 may have a slight edge over a 7-08 given equal pressures, but it aint much and youd need a real good load to see it.

Well, WesternHunter, it's because the 308 doesn't go as fast as the '06.
Loaded to like pressures, and AEBE, the bigger case will give a higher bullet speed.
The 308, 7mm-08 and others may be more efficient, but that's not the same thing.

Actuqally my Speer book maxes the 7x57 out at 46000cup.

Higher pressure produces more energy from each grain of powder just as it does in a reciprocating engine from fuel.

There may be some benefits from particular cartridge shapes but between those two there is not much. Whatever difference there is belongs on the second page and you liking and therefore aiming the rifle better count for a lot more.

If your concerned about energy then chamber it for the 284 Win. If it's effectivness your after then the .358 Win with a 225 gr spitzer would easily overpower those 7mmm's at any range.

Recoil Rob,
The 7MM mauser's specs have been held to a low level because of all the early mausers (Swedes, etc.) that it was chambered in. In a modern rifle it will eat the lunch of a 7mm-08 with ease. And that can be done by loading to less pressures than is shown the rival in question.

Pressure developed in a case is dependant on the available space. Smaller the capacity, the easier it is to develop a certain pressure. For example, if the case was the size of a room, then you would need a lot more powder to develop 52000 cup than in a cartridge. The 7mm-08 with its high pressure loads is not condusive to long brass life. There is no free lunch.

Rifle magazine did an article about 10 years ago about the 7mm. J Barsness did the work and the reloads were designated for modern rifles and they were far higher than what the loading manuals report. BTW, shape has nothing to do with pressures in a cartridge.

The 7X57 is a wonderful round and will take anything that walks on North America if you do your part. It is more suited for a reloader as the factory rounds are held back. I handload and have 3 rifles chambered in 7X57 and none at present in 7m-08.

Take it to the extreme. The 7-08 vrs. the 7 STW, loaded to the same pressure the 7STW produces plenty more velocity than the 7-08. Why? Case capacity, nothing else.

I think "Expansion Ratio" is a better description than case capacity, but of course the two are directly correlated for a given bore diameter.

To achieve peak velocity, you want to keep the pressure under the bullet as high as (safely) possible for as long as possible.

In theory this is simple physics - but in reality is much more complicated, hence the "science" of internal ballistics....

Best regards,

Bill

It's beginning to sink in boys. The theory is beginning take form in the old gray matter. Thanks for the free school and being so willing to teach, and keeping it civil.

I'm using a M1999 short Action to make a versatile rifle for a friend. He doesn't reload and won't have any interest to. The more I learn I'm thinking the best round for him may be the plain old vanilla .308.

Yeah, it's not sexy but there's a reason vanilla such a popular flavor.

But this learning experience certainly will help me, who has too many rifles already and is always looking to learn more. I can't put my finger on what fascinates me so much about hunting rifles but the stuff I don't know sure makes for pleasurable times. If only my girlfriend was into it..... then we'd have something to talk about over dinner (g)
Regards,
Rob

Bill M said it best. Expansion ratio is a better foot to start on. Case capacity doesnt mean jack if you dont use a good recepie. Why do we fiddle around with different powders and come up with huge velocity variations when we do? For instance one of my Speer manuals shows a 300WM lobbing a 165 grn bullet at only 2809 fs with 60 grns of RLR 15 (thats also a max load btw), why then does it show a 308 with that same bullet and 13 grains LESS of the same powder pushing it to 2812 fs??? Acording to the theorys in here that is simply not possible, I guess Speer should recruit some armchair experts and fix their book.. The answer IS efficency! Just the right bang for the task makes it happen. That is also why a 308 CAN match an 06 with certian loads..

I admit that when loaded right an 06 will always have a "SLIGHT" edge over a 308 though, I just want to emphasize that there is such a thing as efficency with all cartridges and that any 08 based case compares extremly well with larger ones because of that fact. As mentioned, its not the shape of the case that allows this but the burn rate and quantity to bullet ratio.

[ 07-30-2003, 05:07: Message edited by: Wstrnhuntr ]

RecoilRob
For a nonhandloader(poor unfortunate person that he may be)the vanilla 308 is hard to beat, he can even peer into the world of precision shooting by just buying Federal match ammo.
Guy Morrison