Five weapons in the game feature an integrated, undetachable silencer. This includes the H&K MP5SD6, AS VAL Kobra, VSS Vintorez, H&K MP5SD Spec, and AS VAL MOD.3 Spec. Note that several other weapons also feature integrated silencers, but they seem to be detachable. This includes the PMM-12, Glock 17 DEVGRU, DT SRS Mk.2, FN P90 DEVGRU, and Vikhlop.
Integrated Silencers have unique mechanics. According to some tests, the integrated silencers of the AS VAL MOD.3, AS VAL Kobra, and VSS Vintorez do not provide full "Stealth Mode". That is, they do not completely hide players from the Minimap when shots are fired. However, they do seem to provide partial Stealth Mode; together, they have the smallest "Hear Radius" of all unsilenced weapons (they are effectively very quiet unsilenced weapons). The Hear Radius governs the distance that gunshots can be heard from. So, since these weapons have a very small Hear Radius, other players need to be located extremely nearby in order to hear the shots. This makes integrated silencers nearly as good as the real thing, but not quite as good.
Of course, there are statistical advantages to having an integrated silencer; you can effectively have a (nearly) silenced weapon without having all of the downsides of regular silencers such as less damage, less penetration, less effective distance, less mobility, etc. With that being said, a notable downside of having an integrated silencer is that you cannot equip other muzzle devices, especially muzzle-breaks, which are capable of reducing recoil and/or increasing effective distance (at the cost of not being able to simultaneously silence your weapon).
It is likely that the H&K MP4SD6 and the H&K MP5SD Spec both also follow the aforementioned mechanics. From inspecting their weapon models, it seems that their silencers cannot be detached. Furthermore, it also seems that silencer-related Gunsmith Skills do not affect these weapons. On the other hand, silencer-related Gunsmith Skills affect the PMM-12, Glock 17 DEVGRU, and other weapons with integrated, detachable silencers.