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I've had experience with Promtheus, they are damn good and pretty close to what the rating states (dependant on the barrel length of course.)
See about getting a King Arms Reinforced Mechbox from Jugglez (airsoftparts.com), they are about $40. I haven't tried them yet but I have four on order (two for a client, one for my MP5SD and one for stock)
Piston, stick with a polycarb one, it'll be nicer on your mechbox.
Piston head............. I've seen everything form 10fps to 30fps droppage in velocity.
If range is important to you (looks like it considering G3 as sniper rifle) you should go with what was mentioned above, get a 120 spring that is trustworthy, then focus on getting parts that'll make the airflow more efficient, such as piston head, cylinder head, nozzle, good hop up rubber (KN RH75 would be good for you, it's rated for guns 400+fps), and a tightbore (adds about 10fps). And use heavy BBs, no lighter than 0.28g.
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