Looking in BIS's commented config files, I see the path to the sound for the m21 is this:
Weapons\m21
I think if you define a new sound, and put that as the file for the sound in your description.ext, than you might get the BIS m21 sound. If that doesn't work, try:
\Weapons\m21
Now, if you don't know how to define sounds in your description.ext, or play them in your scripts/triggers, then I suggest you search the ed depot and read Bloodmixers sound tutorial.