This was posted by Rich in the Yellowforum. I made a huge difference for me too.
I have found out something about this rifle that should be looked at if you own one. I have tuned two of them so far and I just recived my own 350 and all three had the same problem. The breach seal leaks on them, I had one that would not shoot 7.9 CPL's faster than 880's and after the tune it smoked just a little and I saw smoke comming from the seal area. I cut two shims from an old phone book cover and shimmed up under the "o" ring and it shot the CPL's at 1070 fps.
The next one I tuned had the same problem but not as bad only took a single shim from a phone book to get this one shooting good. I just recived a new 350 today and the first thing I did was check the seal with a little powder on the pellet sure enough there was powder flying all over. I found a couple of steel washers that RWS had installed in the groove before the rifle was shipped, mine needed a shim added to get it up to the 1050 range with CPL's. Must be a quaility control issue on the grove for the seal from the factory when they install shims from the factory.
If you own one it might be a good idea to check it, maybe your loseing power and don't even know it..
Another method I have found that works is to use some "plumbers mait" as it is called here in England, this stuff is a very sticky non setting putty used for sealing wastes etc,I noticed some browny coloured goo oozing from the area the barrel enters the breech above the cocking pin on my Diana 38 so I removed the breach seal de-greased the recess thumbed in a little putty pushed the seal back in and closed the barrel to sqeeze out the excess. Result? No more goo + another 30fps.