How much does cartridge configuration play in ballistics?

[Old Time Hunter]

Look'n for ya'll's expert opinions and suggestions. Here is the scenerio, I have two cast bullets that came out of the same mould, both gas checked. But, I load one in a 8X56R shell and fire it out of a M95 Steyr and the other I load into a 8mmX57 shell and fire it out of a Commission 88 rifle. Both have exactly 16.0 grains of Unique...now here is the issue:

The 8X56R only generates an ave of 1332 fps (chrono at 15') and the 8X57 generates an average of 1612 fps. I am stymied! Both bullets weigh the same (209.5 grains w/GC), the 8X56 I size at .3305" and the 8mmX57 at .3245", both of which are nominally .001" over major diameter of the bore. Can it be the case design and corresponding chambers?

[O.S.O.K.]

Yes. The capacity of the one must be larger than the capacity of the other. The pressure generated in the slower cartridge is less than the other. Even if the friction was much greater, it wouldn't slow the boolit that much I don't think.

What about barrel lengths? Are they comprable or is the 88's a lot longer?

[Nath]

The two bore sizes will have an effect.

Nath.

[Hobie]

EVERYTHING has an effect. How MUCH is the interesting part. Have you checked the cases YOU are using for capacity? I bet the 8x56 is a larger capacity case 64.01 gr. water compared to the 8x57J with 62.28 gr. water capacity (according to Donnely). That difference would account for the difference in velocity right there. However, differences in bore diameter, smoothness, twist rates even, could affect velocities.

[O.S.O.K.]

Yes, the bore diameter does make a difference too - that's more volume for the gases to expand in too.

[awp101]

The shape of the case will make a difference as well.

In theory the sharper shoulder and straighter case walls of the 8x57 leads to a more efficient powder burn. That's the reasoning behind the Ackley Improved line of cartridges as well as other "improved"/wildcat cartridges.

[Hobie]

But in this "case" it is likely just the capacity.

[Old Time Hunter]

I think Hobie and Awp might both be correct. Hobie, the 8X56R definitely has a larger capacity it can swallow the entire contents of the 8 X 57 with room to spare. And Awp, the sloping gradual shoulder of the 8 X 56R darn near makes it a modified straight cartridge. Using fast burning Unique, I do not think that the longer barrel of the 88 would affect the velocity much as the volume would equal fairly quickly (3.14 X Rsq) X L. But I'll try it in a 19" barrel K98 I have to see if there is a difference. If it doesn't I'd guess the cartridge design is the culprit.

[awp101]

But in this "case" it is likely just the capacity.

Looks like we'll just have to get down to the "brass" tacks of the situation...

[Don McDowell]

The 8x56 with its larger body and more taper leaves a bit more room for the powder gases to expand generating less pressure than the 8x57 with its smaller body straight case and a more exaggerated shoulder and neck as compared to the 56, thereby generating more pressure. When most everything else is equal more pressure usually means more velocity.

[bdhold]

The two bore sizes will have an effect.

Nath.

I would say bore is everything in this difference.
Powder burn should be close to the same, volume of the hot gas should be exactly the same.

[Old Savage]

Pressure and capacity - and individual barrels can be much different.

[NonPCnraRN]

I think that a good example is the WSM cartridges compared to their conventional counter parts. The short fat cartridges of the same caliber as the longer cases may have the same capacity but the powder burn in the short fat shell is more efficient than the conventional cartridge, all other factors being the same. The WSM cartridges produce higher velocities with less recoil. The only drawback is that magazine capacity is usually reduced by one round. Also the shorter cartridges usually don't handle longer bullets as well as the longe cargtridges. The other benefit of the shorter cartridges is that they can be utilized a shorter action.

[Old Savage]

Higher velocity - less recoil, hmm. I think I would have to see that.

[NonPCnraRN]

Higher velocity - less recoil, hmm. I think I would have to see that.

If I recall correctly a 300 WSM was compared to the 300 Win Mag. IIRC the shorter, stubbier round exceeded or at least equaled the velocity of the Win Mag. Of course I may be suffering from "oldtimer's" vs Alzheimer’s. It would not be the first time I was wrong or remembered wrong. Somebody else may be able to validate or refute the claim. UPDATE: I just checked some articles on the web regarding the 2 rounds. The WSM uses less powder to acheive the same velocity. Recoil with the WSM is less. The original cartridges handle longer bullets better just like the 30-06 handles heavier bullets better than the 308. Bambi will never know the difference. The WSM cartridges are chambered in shorter actions and the rifles weigh less. I you already own a 300 Win Mag rifle you would be wasting money buying a WSM rifle unless you wanted a shorter actioned, lighter rifle. A stated before the WSM cartridges have one less cartridge in the magazine.

[KirkD]

A lot of good reasons given above. Just to add an example, this past week I fired two loads. Both were precisely the same, except on was using cast bullet sized to .312 and the other .313. Just one thou difference, everything else the same. Average velocity for the .312 was 1,260 fps and the average velocity for the .313 was 1,193 fps. I figure .001" difference in groove diameter could also make a difference like that. The same load in Ken Waters' rifle gave 1,376 fps. The last thing I would expect when using the same load in two different rifles, is that they would have the same muzzle velocity. I don't even expect to get close to the published loads.