Hey aldo, more time now!
Really take the time to consider the ICS m4 (because of split gearbox design) that design allows you to switch the upper mechbox (spring and piston) in less than a minute
What you do is buy an extra mechbox an you can go from 325 fps indoors to 385 fps outdoors in seconds (check the video on you tube)
Maintenance on that gun is also breese Nothing is easier to maintain
As an example, After a game in sand, you remove the upper mechbox crack it open clean the parts in the sink with liquid soap, dry, regrease and reassemble in about 20 minutes.
Plus because you can split the gearbox you can see and act on anything going on in the lower gearbox, everything being visible you actually look at parts move! (not so with any other brand)
Since you said under 600.00$ that leaves elbow room..
you could get a full metal version of the M4A1 about (it comes already equiped with a turbo 3000 motor (noisy but bullet proof), metal spring guide and metal bushings.
The only weak spot in the system is the piston that will eventually break (use an aftermarket piston, but you'll need to cut the hook on the spring) or also switch the spring with a non linear aftermarket. (the good news is replacing a spring on an ICS M4 takes about 10 minutes the first time... less than 5 after that)
or as a buddy of mine did you buy 2 standard one for about 289$ and if one should ever crap on you during a game you just grab the spare and fixe the first one HOME not on the field.
Dont let anybody tell you that to get a decent aeg you should spend XXXX$ the most memorable failure i've witnest were on very high sticker items.
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It ain't the years, it's the miles.