13.7" but you'll regret adding that 1/2lb of dead weight on a 16" carbine.  Probably the dumbest configuration in the AR15 world...

The KX3 is a design rip off of the Krinkov muzzle device designed to enhance back pressure and redirect concussion away from the shooter with the 6is inch 7.62 AK47 barrel.
People started playing with them on SBR AR15/M16 hoping for enhanced reliability and to avoid the deafingblast.  They do redirect the blast, but after MUCH testing/evaluation, the back pressure reliability enhancement with an AR is basically nonexistant -its bullshit, no one can prove it actually "adds anything".

Since the on-line world configures their weapons based on picture threads instead of actual research, everyone started sticking them on their guns.

the CAR gas system has too much dwell time to begin with, so even if you believe you are getting added back pressure - its not a good thing.  Putting these things on a barrel permanantly to bring them to 16" is just fucking ignorant.  We pin them all the time, and usually get 80% of them back a month later to remove them.

There is NO db reduction, so dont think you can get away without hearing protection.

For the length and weight from adding them to a 7.5" barrel, you would be much better served from a functional/realibility aspect with a 10.5" and an A2.

If you want to do it right, buy an OPS INC M4-S.  If you dont want to pay the NFA fees because your AR is just a range toy do whatever you want that makes you happy.