OK OK OK You've talked me into it. I've put some thought into how to do it. I will start making the cutter. It will be a piloted single step unit to do the interior in one pass. The check seat will end up just behind the O-ring groove. They will use your factory check poppet. I will make a 4-5 in brass and 6-7 in aluminum for the first run. You will lose a little interior volume, but with my 766 that does not seem to be a problem, with the other mods done to it, it's pushing CPHP's at ~850 with 18 pumps.
For the piston, you buy a 13xx flattop unit from your favorite supplier. When you have that in hand, you go to Ace, Fastenal, Tractor Supply, or your local bolt shop, and get the appropriate size all thread for the piston. Also get a matching "connector nut" (one of these, the long one) Use the all thread and nut to make the piston the right length.To get over ~750 fps will require, a pin probe replacing the magnet (no more BB's), O-ringing the breech end of the bbl (a' la m-rod), and a lighter valve spring.