Check the settings for these Sound Sets:

• Mk20_Tail_SoundSet
• Katiba_Tail_SoundSet

for these classes was added parameter customCategory="limit2"; which apparently completely destroys any second sound shader of these weapons. This is strange because there will be more than two this assault rifles on the battlefield in most cases. Indeed, if these rifles are fired together, glitches in sound will be clearly audible → https://www.youtube.com/watch?v=TtWFKjtA6Ns

No any other weapon has this parameter, check other classes, for example: Rifle_Tail_Base_SoundSet, ADR_97_Tail_SoundSet, etc…

customCategory="limit2"; must be removed from Mk20 and Katiba echo sound sets, result of this you can check in this video → https://www.youtube.com/watch?v=92JQMwXnIug