For the SL pumps, you're basically looking at the Phantom. Chop off the threads, turn the body down a tad, bang, you're done.
Just about everything else out there will also need additional body reduction to fit through the pump. And I can't think of any two-piece barrels that would work.
Now, you can bore out the pump a little, or replace it entirely with a custom-built undercocker setup, but that's not exactly "easy".
