This is the stock spring that comes in most (if not all) G&G AEG's.
I have had two AEG's with this spring, one was the G&G M14, which chronoed a consistent 323 FPS... except for once, when it chronoed 324 FPS. Big deal. I used the M14 for probably a good 9 months and 15,000-20,000 rounds, and the FPS never deviated.
The other AEG I've had this spring in is the G&G FS-51, which was chronoed by the Airsoft GI staff at 330 FPS. I did not have a chance to chrono it again because I switched out the spring for a stiffer one.
I compared both of the springs side by side to check for any loss of length, and the one I had used most (the one from my M14) was exactly 3mm shorter than the almost new G&G M100, which had seen maybe 1000 rounds of backyard target shooting. Most average springs compress anywhere between 10 and 20 mm.
I compared the lengths of
-Did not lose length after heavy use.
-Fairly cheap, but well made.
-Provided a good FPS rating even with a stock gun. In a gun with full compression upgrades and a TB, you could probably hit 350 FPS with minimal stress on the internals of your AEG.