Richard, the barrel is actually stamped 5.56NATO so it will handle both 5.56 and .223 no issue Sir.

it is a .223/5.56 upper and can shoot both "Specifications for Windham Weaponry UR16LHB Upper 223Rem/5.56 NATO 16" No Handle AR-15 4150 Steel:
Mfg Item Num:
UR16LHB
Supplier SKU:
84211
Category:
UPPERS
Receiver Finish:
Black
Type:
Complete Gas Impingement
Caliber:
223 Remington/5.56 NATO
Sights:
A2 Front
Barrel Length:
16.0"
Barrel Material:
4150 Steel
Twist:
1:9
Barrel Description:
Heavy Contour
Barrel Finish:
Black
Receiver Description:
A3"