I just thought of a viable solution to this problem imo which could be applied when machining the square corners of the mag well on an AR-15 receiver or similar slots.
The falling block action will need to be made a little bit wider to keep the strength the same.Rough out the majority of your dimensions of your rectangular slot.Then use a long 3/16" or 1/4" endmill to clean up your corners and machine to your final dimensions.The corners will have a radius which will be less likely to crack.
Then add the appropriate thickness 3/16" or 1/4" hardened and ground flat stock to the two sides(left and right of action) of your rectangular slot.Two screws loctited to hold the flat stock in position should be sufficient as there shouldn't be any force on the flat stock.You might try low temp silver soldering the flat stock if you don't need to heat treat your action after machining.
For test firing try laying the rifle with the butt inside an old tire.Lay sand bags across the action and barrel to hold it down and prevent any shrapnel in case it blows up.Fire one shell at a time at a safe distance with a string to pull the trigger.
Note any signs of incorrect headspace,high pressure,firing pin length,case separation,cracked necks,primer leakage or puncture,brass flow,primer pocket enlarging,primer flow into the primer pocket,offset firing pin strike,depth of firing pin strike,lengthening case,case dimensions before and after firing.
Keep notes and stop if anything looks questionable.You only have one head,think safety.Read all the gunsmithing books that you can and start with a 22LR as it would be more forgiving if something goes wrong.
Even a 22LR makes a horrible noise when a case separates.I had an old worn out Remington 510 bolt action have a case separation.I couldn't hear for a couple of minutes afterwards and I had a lot of background ringing for a few hours.My glasses had a piece of molten brass lodged in one lense and I had to pick a few slivers of brass out of my trigger hand.
Be Careful!!!!!!
[This message has been edited by Barry Briscoe (edited 11-08-2002).]