The best replacement for mp40 is thompson.

The best alternatives to mp40 are rated as following:
★★★★★  thompson
★★★★  ppsh
★★★  mp5
★★  stg 44
★★  ppsh 41
★★  glock 22
★★  wp40
★★  stakeout
★★  mp38
★★  sten
★  tommy gun
★  type 100
★  tunze
★  ak74u
★  tunze 6095
★  grease gun
★  mp9
★  xd40
★  mp 10
  2 mp10
  mp44
  m3 grease gun
  mp10
  sd40
  ppsh-41
  thompson call of duty
  agm mp40

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Speaking about mp40, one probably shouldn't forget about juggernaut.

Other search terms related to mp40 and their ratings are:
★★★★★ juggernaut
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Google Trends

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thompson
ppsh