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Like others have stated it's probably a spring that has remained compressed for too long. Having a better air seal around you hop-up feeder tube could help as well.
I have the same CA M15 as you. I recently replaced the spring with a Modify S100 (rated at around 360-370fps), Modify cylinder/cylinder head/piston/piston head/nozzle, and a KA hop-up (with O-rings). I think the better nozzle and KA hop-up contributed a lot to my increased velocity. It chrono'd at yesterday's FTF game at 390fps.
The O-rings on the KA unit help to make a better seal at both the mechbox/nozzle and the BB feeder portion. Before installing these parts I fed a single BB into the stock hop-up, covered the feeder portion with my finger and fired. That seal I created with my finger resulted in a 10-15fps increase when run through my chrony. That's why I focused on better air seal when I performed my upgrade. Your mileage may vary of course depending on the mags you're using and how well they sit on the hop-up.
If you want to swap the spring and happen to go with Modify springs, keep in mind that the number/meter ratings they use are conservative. As an example, a Modify S100 is comparable to a 110 for other brands.
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