The best replacement for m40 is m24.

The best alternatives to m40 are rated as following:
★★★★★  m24
★★★  r700
★★★  m50
★★  m1
★★  m42
★  m1 to birmingham
★  m65
★  l96 battlefield 3
★  m98
★  scout elite
★  jng
★  sv98
★  m4
★  cs-lr4
★  m21
★  l96
  m41
  l96 bf4
  xm21
  m 98
  m98b
  barrett
  m60
  canon vixia hf m41

Your alternative for m40 is not listed? Suggest!
Speaking about m40, one probably shouldn't forget about acog glitch.

Other search terms related to m40 and their ratings are:
★★★★★ acog glitch
You have in mind a companion for m40 and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of m40 in all years. Click on graph for more.
m40
m24
r700