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Last year my MP5 SD chronied 387fps with 0.20g, this year at two different games it sat between 265fps and 300fps. I figured I didn't properly uncompress the spring (always end of the day fire it in semi a couple times, but now I've found I have to fire it in bursts and check the nozzle to make sure it's full forward to ensure the piston is forward as well). Anyways, I thought my spring had weakened (a PDI 150% that was a good one) since I've been using my more compact MP5 all year, and seems the spring was fine. My Hurricane reinforced mechbox on the other hand, was seriously cracked on both sides.
You might have a cracked mechbox, take off the upper receiver and look at the front of the mechbox to see.
If not, could just be your piston's O-ring has worn out or just needs some fresh grease applied.
It COULD also be your hop up unit, I troubleshot an AK earlier this year that had a large fps drop, I was baffled by it, then though to check the hop up rubber. I cycled it to make it sit with the nozzle full forward, shined a flashlight into the magwell and looked down the barrel to see a sliver of light coming through the bottom. I pulled out the hop up unit to find the plastic C clip had broken in two places. That C clip holds the barrel in the hop up unit, if that clip cracks the barrel will slip forward a little bit every time the gun is cycled, which will end up causing large fps loss.
Hope your problem is one of those, good luck!
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