[Ibby]

I have 2 ICS M4's and a TOP M249. The M4's are very robust and come full metal. The only real upgrades you need to do to them would be a new spring and bushings. With a Guarder 110 spring and bushings you'll get around 390fps. Then you can add whatever else you want. The split mechbox seems to annoy some people, but I absolutely love it. Very easy to work on and swap springs out of if you change game environments.
The TOP M249 is a good gun, but it uses a bellows system instead of a piston. The first upgrade that needs done here is to pitch that and replace it with either an ANGS or Guarder cylinder, polycarbonate piston, and preferrably a Guarder mechbox bottom. The way the mechbox is designed is absolute simplicity, and it will spew forth many many more BBs than a V2 or 3 mechbox before breaking. I bought mine well used and put God knows how many bags of BB s through it. It's truly a hose, and has taken everything I've thrown at it. The only real drawback is that its all metal and heavy as hell. It's currently up for sale, but I'm thinking of pulling the sale, I love it too much...