Probably no major difference, but from everything I have read FN is prohibited from contract by using the same exact processes on it's non-US military M16 variants while Colt is not. Look up information on Technical Data Packages for more details.

"My message to my troops is if you see anybody carrying a gun on the streets of Milwaukee, we'll put them on the ground, take the gun away and then decide whether you have a right to carry it." - Milwaukee Police Chief Ed Flynn