[Freeze]

When you say o-ring did you mean the piston head o-ring? I had a similar problem on my SCAR and replacing that made my FPS hit 360 with a 0.2g bb on a M100. I tried stretching it at one point, but that only lasted for a week and then it was back to garbage compression. I went with the Blackblitz X-Ring, works wonders for me.

My current M4 is running full SHS compression parts with a Blackblitz X-Ring, SHS 12:1 gearset, Modify M100, and gives me 370 +/- 2 FPS on a 0.2g bb. My setup isn't short-stroked though so that may make a difference (I'm not sure about this)

I'd suggest a full run down on every connection point in your compression parts to figure it out. Do it before you try to fix anything.