Hello dear community ot there. This is my guide on how to use the MP5k efficiently. This is based on Callofduty4's, and A Lonely Nomad's guides to the MG4 and LSW, respectively. Because of that, I will use a similar rating system.
Recoil - High, almost uncontrollable with Rapid Fire.
Rate of Fire - 850 RPM, AKA High
Rapid Fire - (4/10) Not very efficient. The low ammo of the MP5k, unless using Scavenger, makes ammo consuming very difficult, coupled with the fact that the MP5k has already high recoil, putting this as an "attachment" will make it uncontrollable.
Akimbo - (6/10 to 2/10, depending on map) Not useful, as the recoil is high, and the crosshairs are very big, unless using Steady Aim, although they still are big. However, if used correctly, it is possible to demolish whole teams on close maps such as Skidrow, athough ammo consumption is still VERY high, making Scavenger needed.
Red Dot Sight - (8/10) A good attachment for this gun, although not really needed as most players will rush with this gun, however, if hanging back, this is very useful as the visual recoil is a little bit less.
Holographic Sight - (8/10) See above.
'ACOG S'cope - (8/10) Very useful if used correctly, horrable if used wrongly. The high recoil makes getting headshots almost very easy, although the recoil sometimes goes left or right. Great alternative to the RDS or Holographic Sight.
Thermal Scope - (3/10) Not useful at all. Firstly, the MP5k is a SMG and not a ranged gun, secondly, it gives no zoom at all, and thirdly, the recoil is VERY high, so if you kill someone, it was pure luck. Skip over this.
FMJ - (5/10) Not very useful, as most players do not "wallbang", and it is only seen used when trying to unlock Extended Mags. Skip over this.
Silencer - (9/10) Excellent attachment. This lowers recoil, has no damage reduction, and makes you invisible on the minimap. Almost a must.
Extended Mags - (9/10) Very useful. The fact that the very high RPM consumes ammo very quickly, making many reloads needed, makes this a perfect perk for this gun. If you have unlocked it, use it.
Marathon - Very useful when rushing, and also allows for a quick getaway if reloading is needed.
Sleight of Hand - Very useful. Frequent reloads are needed, although the reload time is already quick. This makes reloading EVEN quicker.
Scavenger - High ammo consumption makes this a must when not rushing, thus not dying so frequently. Excellent for people who hang back.
Stopping Power - Low ammo and low damage makes this a must. This also makes the MP5k a 4-hit kill at any range, or two if the shots land on the head.
Lightweight - See Marathon.
Cold-Blooded - Excellent in conjunction to the Silencer. Makes you also die not so often ;)
Steady Aim - Excellent. Most players will hipfire with this weapon, and this increases the accuracy, making getting kills easier.
Sitrep - Very useful when rushing and pointing out campers and snipers, as most of those will have a claymore near them.
My first guide, appreciate any feedback