I wouldn't personally buy the Systema. IMO they're really overpriced, and I've seen enough Systems pistons and tappet plates break after short use that I won't pay that kind of money for something that will need work shortly thereafter. I've put a lot of Systema parts in my guns, and more often than not, much cheaper parts ended up being more durable. Although I will say their mechbox shells, gears, and piston / cylinder heads are fine.
King Arms should do you well if you're looking for a preassembled unit that you won't have to mess with for a while. If building from scratch, I'd recommend Modify or Prometheus.
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