Friend of mine bought S&W mod. 60 with MIM(metal injection mold) hammer, serial DAVxxxx a year ago, fired a box of .38spl and noticed inconsistent triger pull (as owner said: every time it is lighter and lighter). After few trigger pull measurement we decided to open it up and see what is wrong.... here is what we found:
and detail marked on previous picture:
Question is what is the possible reason for this crack (fall or incorect use can be excluded)?