If you change your barrel length you may need a new cylinder. Is this for CQB? If it is try to get around 350FPS, use .25g BBs and focus on rate of fire.
Replace your:
probably looking a type 2 cylinder or an MP5 cylinder which will support a 227-430mm long barrel.
plastic bushings with metal bushings not bearings
reshim your gears (if necessary)
systema POM piston head with bearing. You can remove the piston head from a TM piston so use it.
the new spring, the 150% will be easier on your MB and your rate of fire will be descent.
Mosfet switch will up your batter life and ROF. They are cheap too, well worth the money.
http://forums.airsoftmechanics.com/i...hp?topic=439.0 (Tell Frank Matt Lawson sent you, he is a great guy)
If you can fit a 9.6v you could use torque gears on the 170% or use some high speed gears. I would not expect it to last long though. I like 6.03 Prometheus and 6.01 DB customs barrels. A mp5 QD, quick disconect, silencer should work on your preexisting muzzle device.
Anything beyond this can weight until you break something. I would strip my gears first. I got 50,000 on my TM gears with an M120.
mateba