.223 and 5.56 chambers can be different.
5.56 ammo has a wider spec on the case size for what is acceptable to help keep the ammo cost down and production flowing.
The .223 chamber can be made to a tighter spec and so more accurate shooting than the 5.56 which is made to work even when a little dirty at the price of a small amount of MOA.
The chamber on many .223 bolt action guns is tighter and so some 5.56 ammo may be difficult to close the bolt on.
That said, all the major gun Mfs' use the 5.56 chamber on their semi autos, even Ruger.
Who has always labled their Mini-14 .223, except on some MIL/Police models.
Guns that don't use the 5.56 chamber are specificly listed as having a target or match chamber, except Colt.
Colt calls it's nuetered AR15 a "Target Match" even though all they did was cut off the flash suppressor and bayonett lug. The chamber is still the looser 5.56 one.
Clear as mud?
Long story short. Your USR uses the 5.56 chamber like 99% of the guns out there, but is labled .223 to help keep the anti-gun types from crying. :roll: