Classic Army M15A4 X-series problem
 

I have a CA M15A4 U.A.C. (X-series) and I have a low power with a vibration noise in the gearbox. I opened the gearbox and found nothing suspicious except the piston head not compressing all the time in the cylinder. Problem with the gun is vibration noise like a sheet metal that vibrates and low power 280-290 fps with .20g. This gun should shoot 380-400 fps with .20. I have also a lot of misfeed. If I take the parts in my hands, when I put the piston in the cylinder sometime it have good compression and sometime no compression at all. The vibration noise seems to come from the spring because I dismounted the gun completely and just the gearbox in my hand powered by a battery and I hear the noise in the gearbox and likely from the spring. The gun is brand new and no mods. I only replaced the hop-up with rubber and the problem is still there. I suspect: The spring is not good, the piston head is out of tolerance or piston head o-ring is not good. I ordered a new spring systema m120 to see if it can fix the problem. Any suggestions ?

My aeg has been fixed, replacing the spring by a systema m120 was the solution.

FPS result: 353-357 fps with .25g