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Old October 9, 2013, 11:24 AM   #52
Unclenick
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Join Date: March 4, 2005
Location: Ohio
Posts: 21,018
Windfall,

Several points:

1. The .300 Win Mag has more difference in capacity between different brands of cases than any other cartridge I am aware of. QuickLOAD even lists different case brands as if they were different cartridges because of that large difference (the only cartridge for which it does this). The spread is something like 7.5 grains difference in water overflow capacity between brands, ranging from 88 grains to 95.5 grains. This is one cartridge where you want to follow published load data, including choice of case brand and primer brand.

Even though your Norma cases should be on the high end of that range, I would measure it. Manufacturers have been known to change designs without telling anybody. Weigh a fired case with an intact primer. Fill it with water level with the case mouth (no meniscus) and weigh it again. Take the difference in the two weights. You should have around 95.5 grains of water if the case is 2.620" long. For any other length, adjust your result to that length by subtracting your length from that length and multiplying the difference in inches by 18.8 grains per inch (this is for .308" diameter), then adding the result to what you measured. If your case is longer than 2.620", then the result will be a negative number which will reduce the result when you add it.

2. Federal primers seem to have cups on the soft side of the range. You might try a CCI primer and the problem may just go away.

3. On that plot I've previously posted: it comes from Dr. Lloyd Brownell's study done in the mid-60's. Note that the bullet is round nose. The shape of that curve will vary with bullet shape. With kind permission from Jim Ristow, the following is an example of actual pressure measurements showing just 0.030" seating depth difference causing a 20% difference in peak pressure with a more pointy bullet shape.



Dr. Brownell's theory was that the pressure differences with different bullet distances off the lands is due to the amount of gas that bypasses the bullet while it is jumping into the throat. The further the jump, the more time gas has to do the bypassing. This is not a large volume of gas, but it does cause a short stall in the pressure rise. Add to this that a moving bullet engraves deeper when it hits the lands, then goes on to get moving again sooner and expand the combustion volume more by the time the pressure peak is reached. That results in lower pressure.

The sharper the bullet nose shape, the further it has to travel to complete obturating the bore, which is an important reason for the affect of bullet shape.

4. How big a step you need to make in seating depth to get a change in pressure or accuracy also varies with how sharp the ogive is. I'd suggest reading Berger's own recommended procedure for finding an accuracy sweet spot for their long VLD shapes.

5. Finally, you want to check your gun for other variables that could cause pressure issues. Do you have a fast barrel? Velocity is not a great pressure indicator because of chamber and seating depth variation, but if you've got a bullet going 200 fps faster than the manual lists (after allowing for any difference in barrel length), you are running at higher peak pressure.

If you've got copper fouling build-up near the throat, that can cause higher pressure. Carbon build-up in the front of the chamber ahead of the throat can sometime form a ring that causes it. You can slug your barrel to feel for build-up constrictions and to check that its groove diameter is not unusually tight.


steve4102,

That first plot is mine, which fine to use. I don't know about the second one. Since you said it was from a manual, I had to remove it because a manual is copyrighted material and you have to get permission from the owner of the copyright in order to post copyrighted material on this board. Since you don't know where you got it, you apparently don't have that. Please read the board policy on posting copyrighted material. Thanks.
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Last edited by Unclenick; October 10, 2013 at 08:30 AM. Reason: typo fix and clarifications
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