[yuhaoyang]

HA!
they wouldn't... trust me they wouldn't lol...
XYT gears are made of some cheap untreated steel, and the lemon rate is apparently high, I'm not sure if JG mechboxes use CA or XYT gears. CA are of a good quality and will last a long time.
It shouldn't "jam" on semi, if it stops cycling, just pull the trigger again on semi to fire. You can fix this by fixing your trigger finger habits, getting a larger battery, or even a better motor or torque up gears, although that's nearly pointless, just to fix trigger lag, and expensive.
a 6.01 has very specific BBs that it likes; 6.03s can accept almost all brands of BBs, and the consistancy isn't all that bad IMO.
Everything else inside the gun besides the nylon bushings and the piston should be fine; although I really would recommend an AJAX STS system, or a ARS MASK piston and cylinder head. it's I think $45 USD for the AJAX, and $45 for the MASK sorbo stuff, I strongly suspect that the AJAX would do a much better job, but the MASK set helps reduce firing noise (it also requires the removal of the second tooth on the piston); both help reduce mechbox shell stress greatly. Of course, you can use both together.
If you really really are not on a budget at all, you might as well get the Systema M120 from redwolf lol.