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Same as the last two, I will discuss the facts and my opions about the different gun companies.
From my experiences with (my dad's) KWA, they are a great gun for someone who wants a gun that just works great out of the box. Their stock barrel is alright, but if you want the best performance, I would suggest getting a decent one, and you'll be all set to go for the next year or so.
The wiring in the KWA, from what I've seen is pretty good, I've never had a single problem with it, the connectors are on solid, and I'm never worried about accidently letting a large 9.6v 4200mah battery's gravity pull on them. The guns are built pretty solid externally, but can be dinged/scratched up over a lot of use. The RIS on the S-System is one of the only cons I see on KWA. It's not built very solid, and over a few months of gameplay will eventually require you to ziptie it to hold it together.
Internally the KWA doesn't really need any upgrades unless you plan on completely redoing it. In my opinion, it is one of those guns that is fine stock with the exception of the barrel(like on all guns.) The gearbox looks and performs pretty good. Only downfall I've encountered was with an m130 in the middle of sub-freezing temperatures, where the gearbox cracked. I was running my G&G the same day, my dad was using his KWA, we both started firing, his ended sooner. (I was using an m120 and when I took it apart the night after there were 0 signs of wear, the front of the G&G gearbox is built very tough.)
Overall, I'd say the KWA is a great gun for anyone who wants to just get started in airsoft, and not have to worry about upgrading their gun.
KWA Guns do not have bad GB shells, almost any gun with an m130 spring in it shooting in 10 degree weather has a chance of cracking(V2 wise). Don't get the wrong impression here!
*Need someone to write STAR/Ares and ICS, I have had limited experience with those two.