Small update...
Our team had a tech day yesterday giving me an opportunity to try some other springs.
With a Modify 120 spring she shoots 410 FPS (no hop) at 22 RPS. I took the Gate MERF 3.2 mosfet from my M60, which allows for adjustable ROF via the mosfet, and took it down to 70% which gave me a nice 19.5 RPS. The problem, and in hindsight this happened on my M60 as well, is that with the box mag wired in line with the gun it causes feeding issues below a 90% ROF. So I ditched this and tried using different batteries. I did the math and the brick batteries kick out something like 86 amps (max draw) to the motor, so I used lower and lower combined amperage batteries but with really no tangible change. My 3 year old 9.6v NiMh which had not been cared for well from a charging perspective pulled 20.5 RPS... and it has sat for the past 6 months since I last charged it. My conclusion is that the motor is not drawing the full 86 amps from those brick batteries but is operating at or near it's maximum draw with every battery I have. This means I'll get plenty of run time with a single brick battery and there is really nothing I can do to reduce the RPS unless I start adding weight to the moving parts, or get some really low powered batteries. Like 20C burst 1000mah... So I will stick with 22 rps, keep a close eye for any damage to the internals, and use disciplined high rate of fire bursts. The thing sounds like a damn mini gun which I wont lie, is pretty awesome and should scare the crap out of OpFor.
With this settled I turned to the box mag. The electrical tape I used to take up the wobble in the box had been chewed up after only a few times of removal and re installation, so this would not be a long term solution. Further to that after a decent burst I would hear this weird whirring noise from the box which I can only assume was the counterweight spinning after the trigger had stopped being pulled. This may have somehow been a bi product of the active brake on the mosfet. It also wouldn't catch after winding the mag.. after letting off the trigger all the built up tension in the box would unwind which was pretty annoying. I assumed this was just how the box was built.
Anyway I took the whole thing apart and couldn't replicate the whirring issue with it out so I took all the gears out and lubricated them, since there was no lube, put it all back together and tested the box. Strangely enough this fixed the issue. It also fixed the unwinding and allowed the gearbox to build up tension and hold it rather than release.
I took the foam from my GATE mosfet packaging, which works perfectly for this application by the way, and used hot glue to permanently fix it to the 'door' of the box mag. Before gluing it in place I test fit and cut the foam to size. The purpose of this is to completely isolate the BB compartment from the actual mechanical mechanism of the box. Without foam or some kind of separation here, bbs can and will slip through and can cause the box to jam up. Any soft / low density foam will work great for this. 1/2" thickness is ideal. Styrofoam and other high density foams will not work as they are too dense/stiff to form to the shape of the box mag.
A team mate suggested using velcro on the 'wings' of the box mag where it slides into the retaining clip. A small piece of loop side velcro on either side fixed the issue with near zero wobble.
EDIT: We tested the box mag fitment on my team mates Echo 1 (Re branded A&K), fitment of the box mag is identical to Classic Army. In other words this mod should be done to box mags for both CA and A&K
