Shirley’s autopsy very clearly states that she died of blunt impact trauma to the head. Russell’s cause of death is listed as craniocerebral trauma, which is the broader medical examiner term for a severe injury to the skull and brain.
Technically, the term can cover a gunshot wound. In medical examiner speak it's not normally used that way. It's worth noting that even without a skull,
a gunshot to the head will typically leave indicators. Things like metal scrapings, grease, powder, stippling, or fragments would have been noted on Russell's skin and/or clothing.
The evidence
that we have aligns more closely with both victims having been killed by blunt force trauma, likely with the same rounded object. In any case, I don't think it's a detail to get
too hung up on. In regards to the motive, though, it makes a huge difference.