I have a FX Wildcat mk1 in .22 and it’s got the smooth twist barrel.
I use Falcon accuracy plus or jsb jumbo RS, they are basically the same pellet....
Don't know of any other stock that'll fit. I do know the the compact won't fit as the cylinder is shorter than the standard Express. Mate just bought...
I have no experience of this rifle and no knowledge to input re using wadcutters with a magazine rifle, but would be trying pellets from the JSB, RWS...