After testing further, it seems what's happening is that the numbers displayed on the HUD are not displayed the same way. For a host we would expect some pellets to hit for 1X, and others to hit for 21X, but for a client the game is taking the sum of all pellets and then displaying the average. So you are getting the benefits of Devastating Attrition but it's being presented in a way that obfuscates the numbers.
Entered this bug into our database.
UPDATE: After heart-felt discussion with our weapon engineer, this probably isn't something we will be able to fix. In order to optimize the way the game sends damage events over the network, the client doesn't actually receive the damage of each individual pellet. As you can imagine with some weapons shooting 20 or 30 pellets multiple times per second, the network bandwidth would scale up very rapidly. While what you have now doesn't look exactly right visually, in terms of GAMEPLAY,...