Here's two reasons it's useful:

• #1: You can take the loading data for a particular bullet and caliber in a loading manual and easily determine the volume of each powder charge listed. That tells you which charge will more nearly fill the case to the bottom of the neck, i.e., have the best potential for accuracy.
  weight of charge x VMD of powder = volume of charge in cm³

From the table we see that a maximum charge of IMR 3031 will fill the 30 Rammerjammer case the most, and a maximum charge of H 4198 will fill it the least—2.4 cm³ versus 1.69 cm³.

• #2: If we know the bottom-of-the-neck case capacity of our case in cm³, we can use VMD and easily determine how many grains of any powder would fill the case to that point.

Here's how to do that:

First, determine the weight in grains of the amount of water needed to fill the case to the bottom of the neck.

In the photo we see a 7 x 57 Mauser case and bullet.

Place a bullet of the same caliber as the case beside the case and with its base at the bottom of the case's neck. Then mark the bullet at a point even with the top of the neck. Fill the case with water, then ease the bullet into the neck, stopping at the mark. As it enters the neck, the bullet displaces the water in the neck.

Remove the bullet, carefully wipe the outside of the case dry, then weigh it.

For me, a 7 x 57 Mauser w/water = 222 gr.

So 222 - 167 = 55 grs of water in the case

Second, divide the water's weight in grains by 15.4 to convert to grams.

So the bottom-of-the-neck case capacity of my
7 x 57 Mauser case is 3.57 cm³.

So to determine how much of any powder will fill my 7 x 57 case to the bottom of the neck, I then:

Divide the bottom-of-the-neck case capacity by the powder's VMD value.

Let's look at two powders from Lee Precision's table: IMR 4831 and H 4831:

IMR 4831 VMD value = .0735
So 3.57 ÷ .0735 = 48.5 grs of IMR 4831 will fill my case to the bottom of the neck.

H 4831 VMD value = .0725
So 3.57 ÷ .0725 = 49.2 grs of H 4831 will fill my case to the bottom of the neck.

Here's an extremely important use for VMD data. If you look closely at the neck of the 7 x 57 case in the photo above, you might notice a slight bulge about 1/3 of the way up the neck. I shot that case with a light load in my M 93 Spanish Mauser Boat Rifle, written about here. Light loads of spherical powder in bottleneck cases can create a localized high-pressure phenomenon called "neck ring."

Much has been written about neck ring and what causes it in a case not completely filled with relatively slow burning powder. I suspect the primer blast spreads the powder granules apart and simultaneously ignites them all at once instead of in a progressive ignition sequence. Whatever the cause, my light load ringed the neck of my M 93's chamber.

I now use a load of 45 grs of H 4831 which has a VMD value of .0725.

45 x .0725 = 3.26

3.26 ÷ 3.57 = .91 = 91% of the bottom-of-the-neck case capacity using H 4831. Almost no air space.

Thank you, Lee Precision.