What is dr eq in shotgun shells?

dr eq = drams equivalent. Black powder used to be measured in drams, which was a measure of the volume of black powder used in a load (basically the amount used). When smokeless powder came along later, both smokeless powder and black powder existed at the same time for a while. A smaller amount of smokeless powder was equal in power to a larger amount of black powder. To keep things simple shells using smokeless powder were referenced to how many drams of black powder would be needed to produce similar results. This continued into modern times. A shell marked 3 1/4 dr eq is roughly equal to a black powder load of 3 1/4 drams. This allows an idea of how much power a shell has regardless of the actual amount of powder that is inside the shell.