Are we talking about the Ruger Mk I? If so, I have the Marushin model. It's not a GBB, it's a Non-Blowback (NBB). It's a great little piece. I think the KJW is the exact same gun, they just copied it.
KWA, I believe is Taiwanese OEM for KSC. I've handled the KSC Glock 19 and it's a fine little piece.
I don't think the MP5-K has anything wrong with it other than the battery size and lack of a metal body upgrade available. Other than that, it's the same as any AEG.
