To substantially reduce the recoil for SMG's you have to put suppressor attachment, but don't attach it when playing. It surely costs an attachment slot, but given the recoil of some SMG's it's better to have that accuracy advantage and leave enemies watching killcams to look for proof that you're cheating. Still other SMG's don't benefit from this and are better off with something different.

• Grease Gun
  ⭐⭐⭐⭐⭐
  
• MP-40
  ⭐⭐⭐⭐⭐
  
• M-38
  ⭐⭐⭐⭐⭐
  
• Orso
  ⭐⭐⭐⭐
  
• ZK-383
  ⭐⭐⭐
  
• Sterling
  ⭐⭐⭐
  
• Ribeyrolles
  ⭐⭐⭐
  
• Sten
  ⭐⭐⭐
  
• Waffe 28
  ⭐⭐
  
• M1928
  ⭐⭐
  
• Proto-X1
  ⭐
  
• PPSh-41
  ⭐
  
• Type 100
  ⭐
  
• Nambu Type 2
  ⭐

Extremely low recoil SMG's like Type 100 & Nambu Type 2 can't benefit a lot from Grip accuracy wise. But you'd be amazed to see how well they do with Extended Mag, Rapid Fire and Quick Draw or Steady Aim. Speaking of no Grip, Stealthy MP-40 with Suppressor, Rapid Fire and Advanced Rifling is just as good at taking enemies down from long range as with Grip and No Suppressor.