pdi 170% = M120
Silent piston head sets DO eliminate the "snap" sound from the mechbox. Like andy said, they also reduce the shock as well, which is a good thing.
I highly beg to differ. The PDI 170 I got put me at 420-435fps with just it, a silent head set, and a spring guide....Explain THAT!
I have no idea why that would fire at that velocity. With the same internals, I was at 380. I think your experience was the exception rather than the rule.
Far, DEFINTELY put a silent head set in your AEG. I have one, and recommend it to anyone. To upgrade an AEG, I recommend metal bushings, new spring (which you have both), and a silent head set. In addition to greater gearbox durability, it has a bearing, which allows the spring to twist freely. Some argue that this adds fps, but I simply did it to alleviate stress on my piston.
While we're on the subject, I also recommend the following:
-bearing spring guide
-air seal nozzle
-teflon full cylinder (greater air volume than a ported M4 cylinder)
-Helical gears (I have super torque-ups, but any will do, depending on spring and battery used). These gears are damn near indestructable - Over 25,000 rounds fired with these gears, and no signs of wear!