The best replacement for carbine is rifle.

The best alternatives to carbine are rated as following:
★★★★★  rifle
★★★★  mid length
★★  assault rifle
★  carbeen
★  mini 14
★  assault rifle bf4
★  rifle buffer
★  mid length ar
★  rifle buffer tube
★  dmr
★  pdw
★  m1 garand
★  dmr bf4
★  mid length gas system
★  h buffer
★  mid length gas
★  mid
★  mid length 16
★  mid length barrel
★  shotgun
  rifle gas system
  rifle ar15
  sks
  ar 15
  repeater bioshock
  battle rifle
  ar15
  middy
  m16
  5.56
  ak 47
  mid length ar 15
  pdw bf4
  mid length 14.5
  98 custom
  midlength gas system ar15
  rifle length
  handgun
  mid-length
  .357 magnum
  357
  mini 30
  garand
  smg
  357 magnum
  pistol
  shotgun bioshock infinite
  needle rifle

Your alternative for carbine is not listed? Suggest!
Speaking about carbine, one probably shouldn't forget about a2 stock.

Other search terms related to carbine and their ratings are:
★★★★★ a2 stock
You have in mind a companion for carbine and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of carbine in all years. Click on graph for more.
carbine
rifle
mid length