Y'know, this mod really makes me think about what can be done with the RFR, Mustang 6, and Clip Tek. If you can remove the Buzzbee shells from the Doubleshot, is it possible to remove them from the other Buzzbee blasters out there? The Mustang 6 would probably end up being better than the Maverick, and the Clip Tek would be awsome. We could probably squeeze 70' of so out of the RFR, and it would all be shell-less! Awsome job, I agree, best Double Shot mod I've seen.
I can't comment on the other blasters but I doubt that breech/CPVCing the RFR would make any improvements - the problem is the tiny plunger - there's no cure for that. In fact, I'm not sure that the ranges would differ from the cpvc shell mod and there would definitely be a loss in terms of RoF