I have seen this happen to a friend shooting a 8x57 Mauser, a similarly configured case to a 308. He got some partial head separations & it turned out he was over resizing them. A combination of max chamber (common in ex military rifles) & a resizing die set right back as per makers instructions was setting his shoulders back a bit each time. Firing pin strike pushes case forward, combustion pressure grips case to chamber walls. Pressure continues to push unsupported head back against bolt face & stretches cases above the solid head. The groove you could feel is the thinned area where the stretching has occurred. The answer is to back the resizing die off till it just sizes the neck without setting the case shoulders back. Some smoke their cases to gauge this, others use maker pens.
Steve