Best option to quiet down FX400 .22

I just bought a nice used FX400, regged with a 500cc bottle on it. It was fairly quiet but when I ran it over the chrony it was only shooting the USB 18 gr at 820 fps. I turned it up to 900 and it has a pretty good pop to it. Turned it back to 875 and it's better but still not quiet enough. 

The factory shroud basically ends at the barrel since the end cap screws onto it. Really hate to go much longer so what have you guys done with your 400' to quiet it down?