Load density is the ratio of the powder charge volume to the available case volume – basically an indicator of how full of powder the loaded cartridge is. This is important to be aware of because generally speaking, a load with higher density (the charge more completely fills the available space in the case) can have higher velocity and is often more accurate than one with lower density. Additionally, since the powder charge cannot shift within the cartridge, ignition tends to become more consistent which can result in more consistent velocities.
At Nosler, we calculate load density using bulk density of the powder. Bulk density is an indicator of how dense the powder is, or how tightly the granules pack next to each other in the case. Powders with higher bulk densities (typically presented in units of grams/ml) pack more tightly than those with lower bulk densities. For example, a 308 Winchester case holds 48.3 grains of water with a 165gr AccuBond seated to 2.800” OAL. A load of 48 grains of IMR 4350 (bulk density = 0.945 gm/ml) has a load density of 105%, calculated as follows:
Charge/(case capacity X bulk density) = 48/(48.3 x 0.945) = 1.05 = 105%
In certain cases, load densities will exceed 100%. This is what is called a “compressed load”, and is no cause for concern. All powder charges have air space around and between the granules, which can be compressed to a certain extent when the bullet is seated. In some circumstances, especially with new cases that have not been fire formed to a chamber yet, load densities may be well over 100% and it may be difficult to get the entire powder charge into the case before seating the bullet. If this is the case, fire your brass in the chamber with a load that fits better, resize the cases, and then use the desired load.
With highly compressed loads, it is important to look for bullet creep over time – the bullet slowly migrating back out of the case. A crimp may be necessary to prevent this, as pressures will rise if the bullet gets near or contacts the rifling in your chamber. Additionally, powder bulk densities can and will change over time and from one lot of powder to another.