>>529226947
>>529227781
>>529228605
Tested the turret and AR in Halo 3 using Crow's Nest as well. These two at least seem to have identical cartridge models.
Still the earliest mention of the turret being 7.62x51 is Frankie's encyclopedia. I saw the source for the MA5C being 7.62x51 was from 2007 back during the Bungie era, but I go to check and lo and behold,
https://web.archive.org/web/20071216064559/http://www.bungie.net/News/content.aspx?type=topnews&link=BAGAssaultRifle
It's fuckin' Frankie.