I think the old rule of thumb is 200fps for every inch. Although I've heard 100fps, 50fps, so who really knows. Using 200fps/inch drop a bullet from a 26" barrel doing 4200fps will be 3400fps out a 22" barrel (4" x 200fps = 800fps - 4200fps = 3400fps...).
Now, would I expect this much drop in velocity, I doubt it. I expect more of a 200 - 300 fps drop out the shorter barrel, but I'm of course just guesing. It all depends on the powder burn rate. If the bullet reaches max velocity in 24" then a 22" barrel won't hurt that much. If 26" is when it reaches it's max, then a 22" will take a hit in velocity.
I guess we'll have to wait and see when someone gets one and shoots it through a crony.
later,
scruffy