The best replacement for m16 is m4.

The best alternatives to m16 are rated as following:
★★★★★  m4
★★★★  ak47
★★★  ar15
★★  m14
★★  famas
★★  ak 47
★  ar 15
★  g11
★  ak
★  ak 74
★  m16a1
★  g36
★  m4a1
★  ak-47
★  g11 black ops
★  nemesis
  scar
  ar15 bolt carrier
  g3
  m4 carbine
  mi6
  m16
  ar 15 bolt carrier
  ar 15 bolt
  ar15 rifles
  m14 black ops
  famas mw2
  famous
  m1
  ak74
  ar15 difference
  mp5
  sa80
  sks
  kalashnikov
  fal
  m-16
  c7
  galil
  m4 cod4

Your alternative for m16 is not listed? Suggest!
Speaking about m16, one probably shouldn't forget about grenade launcher.

Other search terms related to m16 and their ratings are:
★★★★★ grenade launcher
★ jesse kelly
★ m203 grenade launcher
You have in mind a companion for m16 and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of m16 in all years. Click on graph for more.
m16
m4
ak47