I personally own the Echo1/Cyma RPK. It's an absolutely fantastic piece. The stock hi-cap it comes with is pretty crappy, my spring broke pretty quickly, but anRPK looks best with a drum anyway. I can't speak for the MK46, but I've heard the hop-ups are terrible.
Also, the RPK is MUCH lighter and more manoeuvrable. It's a bit long, but I'm 6'4" so I don't mind :P
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