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Discussion Starter #1
Jack -

I used 66.0 grains for that estimate, with a seated 6mm 100 gr bullet, COL 2.8". However, I made an error using a Nosler manual, and the capacity of that case is much greater.

My own fired 300 WSM cases hold 82 grs of water to the top of the neck, and necking it down to .243 would lose not more than 3 grs capacity. A 6mm bullet seated 0.4" takes up another 4.6 grs, for a total displacement of 7.6 grs. Net capacity for that application is 74.4 grs of water.

Hence all of my estimates are about 3.7% low in the velocity department. The 6mm WSM should push a 100 gr bullet 3580 fps and generate over 2800 FPE. However, all this now requires a 29 inch bbl at 65,000 psi. A 26 incher will do a little over 3450, and a 24 incher is just a hot 240 Weatherby.
 

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I have read other experimental data generated with QuickLoad that a 6mm WSM reformed with a 40 degree shoulder and minimal taper would have 74 grains of usable volume with 100-grain bullets. Using IMR 7828 and a 22" barrel, velocity was estimated at somewhat less than 3400 fps. It seems like a whole lot of bother and cost for about 200 fps more than the 6mm Remington. :rolleyes:
 

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Discussion Starter #3
Bill -

This sounds about right. My proprietary program estimates 3582 fps for a 29 inch bbl with the same data. This program computes the correct bbl length for maximum practical efficiency and bases velocity on that.

Barrel length really is the boogy man in internal ballistics calculations - for max practical efficiency in a 26 inch bbl the .243 caliber can't exceed 46 grains of usable water capacity. 74 grains grows that bbl a lot.

If we want to use 74 grs of capacity with the .243 caliber in a 26 inch bbl, we need a .308 bore - yup, a sabot.

This combo will clock 4050 fps. As a check, I actually loaded my 300 WSM to max pressure with Win 760 and MEASURED 3850 fps. A 100 grain bullet would do 4037 fps with a slightly faster powder.

To maintain 4037 fps in a 22 inch bbl, we would need to go to an 85 grain bullet, and increase burn rate. Or, we could be happy with the 100 gr bullet @3700 fps from the 22 inch bbl.

Instead of all this experimentation with necked down cases, we need to start using sabots! They are also capable of setting accuracy records, but that's another story...
 
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