The best replacement for 9mm is .40.

The best alternatives to 9mm are rated as following:
★★★★★  .40
★★★★  45
★★★★  40 cal
★★★★  .45
★★★  380
★★★  40
★★★  9mm luger
★★★  38
★★  .380
★★  357
★★  45 acp
★  10mm
★  22
★  357 sig
★  40 s&w
★  .357
★  .38
★  .40 cal
★  .45 acp
★  357 magnum
★  .40 caliber
★  40 caliber
★  38 special
★  40sw
  .45 acp ballistics
  45acp
  45 cal
  .22
  .38 special
  22lr
  .40 s&w
  45 caliber
  40 s w
  357 mag
  .22 caliber
  9x19
  .40 s w
  .45 caliber
  38 super
  40 stopping power
  .380 acp
  380 acp
  40 mm
  40 recoil
  .40 cal ballistics
  45 price
  45 acp ballistics
  .40 stopping power
  .38 caliber
  22 pistol
  10 mm
  .223
  40 cal ballistics
  .45 cal

Your alternative for 9mm is not listed? Suggest!
Speaking about 9mm, one probably shouldn't forget about suppressor.

Other search terms related to 9mm and their ratings are:
★★★★★ suppressor
You have in mind a companion for 9mm and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of 9mm in all years. Click on graph for more.
9mm
.40
45