First mp40 was recorded in the woods where there are many trees that mp40 sound can bounce off of, which would make gun sound louder than open area with no objects.
Second mp40 was recorded in rather open area and there's heavy wind blowing, making mp40 sound much weaker as the heavy wind sound is already interfering with mic recording.
If you go to range and fire any weapon, you'll know that real guns have really loud sound with 140 decibel level.
140dB is one of the loudest sound you can hear in life and is the level of sound that can deafen ppl if exposed for a long time.
Most games have really weak decibel level of gun sound which makes them very gamey, which I think was implemented to protect ears of gamers.
Acorrding to a chart below gun has 140dB, and even short time exposure to that decibel level of sound might literally hurt ears.
While games as RO1, crysis 2, BF3 might be regarded as relatively successful presenting rather realistic gun sound, ro2's gun sound is one of the most disappointing factor of the game.
For correct form of decibel chart
http://www.gcaudio.com/resources/howtos/loudness.html
Decibel (Loudness) Comparison Chart
Here are some interesting numbers, collected from a variety of sources, that help one to understand the volume levels of various sources and how they can affect our hearing.
Environmental Noise
Weakest sound heard0dBWhisper Quiet Library30dBNormal conversation (3-5')60-70dBTelephone dial tone80dBCity Traffic (inside car)85dBTrain whistle at 500', Truck Traffic90dBSubway train at 200'95dB
Level at which sustained exposure may result in hearing loss90 - 95dBPower mower at 3'107dBSnowmobile, Motorcycle100dBPower saw at 3'110dBSandblasting, Loud Rock Concert115dB
Pain begins125dBPneumatic riveter at 4'125dB
Even short term exposure can cause permanent damage - Loudest recommended exposure WITH hearing protection140dBJet engine at 100', Gun Blast140dBDeath of hearing tissue180dBLoudest sound possible194dB