So I revisited the crappy M4 sound effect that I made. I realized the reason it didn't sound right wasn't because of the lack of reverb, it was because you couldn't hear the gases escaping. So I used some brown in pink noise with reverb and reverse bouncing ball delay to approximate the sound for gases escaping. I also show what three round burst sounds like and what it sound like with reverb.
I mixed all of this together in Audacity from scratch. At first I started off mixing in some library sound effects from Counter-Strike, but they are inaudible and almost completely gone. What you hear now is all me and no one else and no recordings, all synthesized from scratch in audacity.