My guess would be 500 if it is BROKEN, or if it is using some pellet made from mercury. I think you can get it to shoot 1100 with really light pellets, but what's the point? What matters is what gets pushed out. I recently chronied my .177 @ 830fps, but with 10.5 gr pellets. If you use the equation 1/2MV^2=E, you can convert to what it'd be shooting with a more 'standard' 7.9gr...It'd be 956fps, which is pretty accurate according to other chrony tests that Gene did when he tuned the gun. With 4.8gr crossman silver eagle hollowpoints, that would be about 1225fps...Maybe you'll loose a little in the conversion, I don't know, as I don't buy pellets that are that light, and really think breaking the sound barrier can affect accuracy, and I wouldn't go lighter just to gain speed. From my experience, heavier pellets are slower and more consistent.
Why are they 'hard to find'? Just go to compasseco.com. They have plenty of them. If you wait for a 10% off sale you can get it for $134.