Everything is possible, just need to find out how. I think it's in one of the .FF files, not sure. In cod4 the gunsounds are there.
If not you can follow these steps:
Extract the m401a gunshot sound from one of the IWD files in CoD4.
Go to mw2 folder, go in MAIN, search in all iwd files till you found an intervention sound file.
Extract that somewhere.
Look at the name of the intervention, if the name is like "intervention_shot" (just an example) then copy that name and rename the m401a shot file. So both files have the same name. (intervention_shot.wav)
Download crcfixer (texture modding section, use search button). Open it, after the "original" type the location of the INTERVENTION sound. In the "new" type the location of the M401a location.