is it for Mp5 replacement ?

I don't know if it's me, but once I reload the gun (the reload animation ends), it's not until something like a second later that the zero is removed and replaced by the reloaded ammo. That is, the animation ends but the weapon still has zero bullets. This makes, for example, one believe that at the end of the reload animation the weapon is already reloaded so perhaps you switch to the secondary weapon or something else and then, when you need to use the primary again, you will realize that the character does the reload animation again. I don't know if I explained myself well, but it happens with all your weapons XD

It may be a bug or the reload animation is faster than the game's original reload animation. :-v

the sound of it is so epic!

This could fit Silenced SMG. The muzzle thing can be seen as a supressor and it's a laser gun.