The best replacement for 45 acp is 9mm.

The best alternatives to 45 acp are rated as following:
★★★★★  9mm
★★★★★  45 gap
★★★★  45 auto
★★★  10mm
★★★  45 colt
★★  40 s&w
★★  357 sig
★★  40
★★  45 long colt
★  357 mag
★  357
★  44 magnum
★  gap
★  44 mag
★  357 magnum
★  40 cal
  45 lc
  auto
  44 special
  45
  40 s w
  9mm ballistics
  9mm which is better
  .357 magnum
  10mm auto
  40 s&w ballistics
  .357 sig
  .40
  38 super
  40 caliber
  .40 s&w ballistics
  45 auto ammo
  38 special
  .40 s&w performance
  .44 magnum
  .44 mag
  10 mm
  10mm ballistics
  colt
  9mm recoil
  40 sw
  7.62x25
  45 long colt ammo
  9 mm
  40sw
  sig 357

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Speaking about 45 acp, one probably shouldn't forget about bullseye powder.

Other search terms related to 45 acp and their ratings are:
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