18-day Necro, but I had some info, being a FAMAS owner myself.
Considering you have an EG-550, I'm assuming you have a TM FAMAS. As such, when it comes to internals, you're pretty much out of luck. The original TM's V1O (Version 1, Old) Gearbox used massively huge amounts of proprietary parts. Everything from the spring guide to the gearset is all V1O Property. The only thing I can forsee being exchangeable would probably be the piston and the Cylinder, but even those I would be hesitant on.
I know Redwolf has parts for the V1O, and I remember a site somewhere that sold only FAMAS parts. I'm gonna have to find it for you. But for now, I would say that there is nothing compatible with the 'New World' from the Older V1. Even the V1N Gearbox used in the JP, CYMA, and CG isn't related to that one.
Sorry.
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