I would tend to agree, 7.62, .30 caliber. This being said by someone on both ends of one for 1000s of rounds... Also would explain the number of casualties. What’s cover for a 5.56 is merely concealment for a 7.62.
Another poster stated the rates on each. The 7.62 AK is flat rated at 600. The AR, not mentioning the round size is 700 to 900. I would assume the 7.62 AR would be the slower and going to some audio of each, it is tough to tell. I would have to go into some audio analysis software to compare the noise spikes on a time chart to actually determine. What I was hearing in the thud verses the echos was the sound, not so much the speed. There are two distinct noises of the noise for each round. What most were hearing were the echos.
To any event, we will know sooner than I can find the time to do the audio analysis unless someone else here has the capability. An old audio editing program called Tsunami Pro was good for this.