? in ??? bayonet m7 m9 fittment

Just wondering, i have a 16" barrel and was telling a friend that a bayonet would not fit on this barrel config. just wondering am I correct to flash hider sits to far from the lug , i thought only the 14.5 mil spec and 20" barrel would allow this

even tho i think c than d magazine has a bayonet lug that clamps on the barrel

If its a carbine length gas system then the distance from lug to flash hider is too long for a military issue bayonet. However the bayonet will fit on a midlength.