While most forum disputes are about words, I think that @frittig makes a valid point in questioning whether the lawful shooting of active criminal home invaders ought to be considered a "mass shooting" just because there happened to be a lot of home invaders. So, yes it's about words in part, but also about concepts.

For my part, all else being equal, for every N criminal home invaders, the optimal number of home invaders repelled by lawful force without injury to innocent civilians is also exactly N. If you want to mix those relatively rare good mass shootings in with other bad mass shootings (the vast majority of them), that's one way to approach things. You could also keep a list of "Things that happened on Tuesdays in history" and it would be a perfectly self-consistent list.