Some random bits of data from HaloGAF users* on
this list. The least accurate player is Fuchsdh with a headshot percentage of 57% over 312 competitive games played. The most accurate is bananas with 93% over 615 competitive games played. HiredN00bs has the most games played with 7752 competitive games played. Awesome Barlow has the highest headshot rate** with a sniper rifle at 69.7%. The gaffer we don't want to give a sniper is Deputy Moonman, who has only a 42% chance of hitting the head with the long rifle. On average HaloGAF finishes 74.24% of their headshot capable weapon kills with headshots.
Judging by the data collected so far, the game registers headshots pretty well. Shadow really shouldn't have argued that it was the games fault when his accuracy with generic headshot weapons was only 69.5%, a good 5% lower than the HaloGAF average.
*I've only been able to aggregate data for letters A-H so far. I'm working on getting the rest, but my API key bugged out around gui's gamertag.
**There is one gaffer(Dark FaZe) with 75.9% sniper headshot rating, but he has less than 150 games played, which is far below the standard.