Colt branded Black Hills Ammunition
77 grain Matchking 5.56mm ammunition. 20 cartridges per box.
Caseheads have the NATO headstamp, WCC (which is Winchester Cartridge Company) and it is dated 13 (2013).
The Colt AR-15 Resource
"A site dedicated to the history of the Colt AR-15 rifles, carbines and associated items"
The Colt AR-15 Resource 