Do you mean in the 600mm B liner? there is no 600mm pellet A Liner is there? certainly not here anyways, maybe only Australia?
I have tried the 25g Monster redesigns with the 600mm B liner in the Crown and the Impact, velocity was maxed at around 860FPS, accuaraccy was okay but not overly exciting at 25 yards.
Pushing the pellet through the liner leads me to believe it is too tight in the barrel, either this or the .22 transfer port (or both) is limiting the velocity, as expected you get slightly more from the Impact than the Crown.
Certainly as above and not very promising is that the 18.13 seems to perform better than the pellet the B liner was actually designed for!