Try using DisableWhileReloading = true in the weapon blueprint, this is what we do with FAF gun TMD. The issue you describe seems to be that 4 TMD fire, the 5th missile gets locked on to by a reloading TMD out of the first 4, and missiles can only have a number of shooters equal to their HP, so that lockon makes the 5th TMD ignore the missile as its shooter cap is filled.