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Old October 14, 2012, 05:03 PM   #13
Unclenick
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Join Date: March 4, 2005
Location: Ohio
Posts: 10,247
Smuckle,

The limitation of the VMD charts is they assume every lot of powder has the same bulk density as the last. It doesn't always work out that way. Only Western Powders (Accurate and Ramshot), AFAIK, posts bulk density tolerances for their powders, and it gets as high as ±5.6%. That means those standard VMD tables could be off by that much¹. So, you still have to use a scale to verify what you are actually getting.

That said, the tables should get you somewhere in the ballpark and they can reveal what the actual true VMD is for your powder lot. The VMD is just the number of cubic centimeters (cc's) each grain of powder occupies. Since the Lee powder measures are all calibrated in cc's, you need to convert from grains weight to cc's of volume. For that, you just take the number of grains of charge weight you want and multiply it by the VMD. That gives you the number of cc's you'll need to set the measure to (or choose that size disc or choose that size scoop).

Once you have the measure set, you may find the actual throws are off by ±5.6% (or whatever the number is for your powder). No problem. Measure and average about 15 throws (30 is better, but seems to take forever). Divide the number of cc's your measure is set to by the average real charge weight you measured. The result is a corrected true VMD for your particular powder lot.

Armed with this corrected VMD, go back to the beginning. Multiply this new true VMD by the number of grains of powder charge you are trying to achieve. That will give you a corrected number of cc's. Choose a disk or scoop or adjust the measure to match this new number of cc's. This time you should get the charge weight you want.

If you are using the fixed discs or scoops and don't have the exact number of cc's you want (it's usually the case that if falls inbetween two of them), you can divide the number of cc's on the sizes above and below your calculated number by your new, corrected VMD. That will tell you how many grains those two choices will actually throw, so you can select the one you prefer.



¹ Caution: the Accurate and Ramshot sites list VMD's in both metric (cc/g) and semi-English (cc/grs). That is, cubic centimeters per gram and cubic centimeters per grain. The gram version is for European users who weigh powder in grams instead of grains. Just watch that you get the right form.
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Last edited by Unclenick; October 14, 2012 at 05:09 PM.
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