a few years back I was working in a situation where I saw a lot of ballards go by and there was periodical discussion about the "ballard cracks". I never saw one cracked in the receiver ring as shown. I did keep track after a while of every one and some where I have a list with between 35 & 40 ballards, forged and cast, all calibers, some rebarreled, some totally original and 50% had "ballard cracks" meaning off the corners of the mortise for the breech block at 45 degrees angling out and back. these guns were in to be worked on in various ways. most owners didn't even know they were cracked and some were being shot a lot. I love a ballard too much to blow one up and if one has those cracks  i think they will make the best .22 you ever had. but lay off the BPCS loads. old guns were shot with lighter bullets,(England at one time used a rule of 20% heavier bullet in a factory shell if a proof shell was unavailble for reproofing so you know pressures are boosted) , now people use a thou or 2 over groove dia. old timers often used bore dia. bullets, twists are much faster, alloys tend to be harder,bullets are now seated into the rifling sometimes crimped. all this puts alot of  strain on an action that has design limitations. and I would guess 19 out 20 or more of the people who mess with ballards don't fully understand how properly fit them up and correct the many things that arise when dealing with a worn or mixmaster action. sorry for the rant if I stepped some toes, just my.02
kootne

Seems to me that the receiver ring is very thin.  I checked my Ballard and there is alot more "meat".  I'm wondering if gunsmiths or the factory bored out the receiver ring to enlarge it so larger cartridges can be accommodated?  Thereby weakening the receiver.

I took a couple of photo's of the bottom, but they were too dark to see.  I"ll take some more this afternoon outside after I get off work.  The hole underneath is essentially the same as yours, maybe not quite as deep.  I may be able to check it with a caliper to see how thick the metal is there.

dave