Look at the loose piece. This appears to be the forward part of the missing area. It does appear to have a bulge as well as a slight flaring of the forward wall of the break in the barrel itself. This may well have been the result of a loose base wad lodged in the cone with the rupture starting at that area. The force there may have been severe enough that as it ruptured it just tore out the mat'l behind the actual rupture point.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra