[Kozzie]

That looks about the same size as the Prometheus spring I have in mine. I never crono'd the stock spring, I was told that it's above 400 fps which doesn't let me use it at most of the games I attend so I swapped it out right away.

I would try putting the stock spring guide in and see if that buys you the space you need to use that spring. I'm not sure what FPS you're after but I'm getting +or- 395 out of my set-up with no hop last I cronied.

By BU I'm assuming you mean bore-up. Maybe thats the problem? The bore up parts are too big. Do you have a longer barrel installed if not you don't need a bore-up kit.

The RS gearbox is slightly shorter, 5mm I believe, than a V3. This was done to achieve true 1:1 dimensions for the gun. As a result the piston/sector gear have one less tooth. I think everything else is V3 compatible. I think it's even possible to use a regular piston and sector gear but the piston needs to be altered to shorten it.

IMO the cylinder head is worth switching to a metal double o-ring version and I prefer bearing spring guides. Other than that the stock RS parts are good parts.