223 Rem means it was loaded to SAAMI specification.
FC17 means it was part of a government contract to Homeland Security, and produced in a civilian plant for civilian law enforcement.
ATK/Federal had taken over the Lake City military ammo plant in Missouri (from Olin/Winchester).
While Lake City was trying to produce enough ammo for wars on two fronts AND modernize the Lake City plant, the Department Of Homeland Security ordered 3.2 Billion rounds of ammo which overwhelmed civilian manufacturers.
In response, ATK/Federal cleared the change for both military & civilian ammo to share the same case materials & specifications for the government contract.
The civilian (DHS) ammo will have mostly .223 headstamps WITHOUT the NATO circle/cross, but mass manufacture headstamps will mostly look 'Military', along with crimped primers.
(You will run into the same thing in .308 Win, the ammunition was loaded to .308 Win pressures, not 7.62 NATO)
I find these same brass in my military range bulk purchases where civilian LE (law enforcement) practice on military ranges. There is ZERO difference between these and newer military brass, same weight, same case capacity, etc.
Keep in mind that Millions of overrun & out of specification rounds of ammunition that didn't make the government cut are still getting dumped on the market, they are bulk packed at gun stores everywhere and show up on ranges everywhere also.
Since the resupply of that massive stockpile is ongoing, the same rounds are still being made...
Since the first of that order was made in 2012, it's nearly time for it to age out & hit the surplus market.