I guess we are reading two different things. The one I posted specifically says M4/M4A1 and M16A3/A4
"FN does this under government license, and this license applies only to the production of the M16A3/A4 for U.S. government contracts. This licensing agreement is actually between Colt and the U.S. government, and it permits the government to competitively procure the M16 rifle from FN."
Even if the agreement covers the M4/M4A1 and the M16 series of rifles, that means the only barrels covered are the 20" rifle and the 14.5", M4 profile barrels with carbine-length gas systems. Change any spec, and it's no longer covered under the agreement..
Change the length to 18", 16", 13.7", 12.5", 11.5", or 10.3" and it's no longer a barrel manufactured by FN for the US military, so no longer covered under the agreement.
Change the 14.5" from an M4 profile to any other profile. No longer a barrel manufactured by FN for the US military, and therefore no longer covered under the agreement.
Change the 14.5", M4 profile barrel from a carbine-length gas system to a midlength gas system. No longer a barrel manufactured by FN for the US military, and therefore no longer covered under the agreement.
Change the twist rate to anything other than 1:7", or change the number of lands and grooves from 6 to something else. No longer a barrel manufactured by FN for the US military, and therefore no longer covered under the agreement.
Change the gas journal from .750" to .625", .825", or .936". No longer a barrel manufactured by FN for the US military, and therefore no longer covered under the agreement.
Change the chamber spec from 5.56x45mm to .223 Wylde. No longer a barrel manufactured by FN for the US military, and therefore no longer covered under the agreement.
There are a slew of ways to get around limiting criteria, and I guarantee the legal beagles at FN know them much better than I. They've been making military arms since 1889.