“It seemed odd to me the writer could give a precise number of recovered cartridge cases recovered, but not a precise number of .223 cases.”
The only source I have seen is from the police department or county attorney. I believe they said that 10 (or 12?) were from .223’s.
And we've been over the fact that not all stoner variant rifles are chambered in .223/5.56X45, too. A .300 blackout case wouldn't be included in the .223 count, nor, for that matter would a 7.62X51 (.308).