Clearly there is very high stress, but without handling one and looking it over for
a while, it is not at all clear why there should be high stress there. Another possiblity
is that they heat treat the slide and lock in some serious stresses from cooling,
and they eventually equalize by cracking.
Clearly their problem, and a bad design. If I was on the team, I would immediately
change the slides to have a .020 to .030 radius in that corner, cheap and easy, good chance
of solving it without fully grasping exactly why it is happening. May require some other
changes to fit, not sure. No doable after the fact.
Inside corners in anything significantly stressed should never be tight, sharp radius. No
benefit in most cases other than easy to run a sharp cornered mill cutter in there rather
than get one ground with a tiny radius, and use it for the finish cut.
I spent a career with people bringing stuff like this (not guns) to me and paying me
to "make it stop". Often I had a good idea right off the bat, at least after the first
decade or so. But long distance, not handing the pieces and looking at them for
quite a while, nothing jumps out. In many cases, the pieces sat on the desk for
a few days, while I studied materials, loading conditions and started on computer
models. Sometimes it just jumped out, but often took a computer model to show
what was really happening. My bet is that they don't have any of that kind of
engineering in house, not common in smaller companies.
I designed, patented and sold a replacement recoil spring plug for Officers ACPs, the
originals broke, and we paid extra for a cutter with 0.010 radius to be specially ground
to avoid this in our interior corner to the flange, highly stressed area. Also, I used
a very strong alloy. Never had one fail, and we have a lifetime guarantee. Still have
a few if anyone still has an Officers ACP with original spring plug. At one point they
asked us for a quote for supplying them, but they wouldn't/couldn't pay what we needed, even
if we upped production quantities masisvely and changed to more efficient methods, so it went nowhere,
They totally redesigned the system for current models.
Bill