ICS L85 not firing properly

[ItsRainbowDash]

What I did was changed the piston, spring, inner barrel and hop rubber. The rifle didn't fire at all, and I've since reverted all parts back to the original ones. When I put it back together, it was firing at around 150 FPS and double feeding on 1 in 3 shots, after about 12 rounds it stopped feeding completely.

I've found that it fires fine when the rifle is upside down, and I've tried it with 4 different mags which have had no effect.

I'm at a loss as to where to go from here really.