After much careful analysis of combat logs, I realised that they don't accurately reflect what is happening in game, and in fact can be quite misleading.
After further analysis and some inspiration, I realised what's going on here and why, and that the guy who wrote it was aware of the problem.
I know it's not trivial and will probably involve some fiddling with the guts of the backend, but please - sort this out. It is extremely difficult to evaluate different weapons and BO powers without accurate data, and what we currently have is not accurate at all. A bug I was hunting tonight was massively complicated by having to work around this.