7.62 is by far the better round, for penetration, distance, knockdown power, blood loss. 5.56 is outdated, and those who have used it in a high stress combat situation will most likely tell you the 5.56 performed, but not the way 7.62 would have, especially when encountering barriers and body armor. Thus the resurgence of the M1A platform, and the SR25 project that continues to evolve.
Of course, I am speaking from a combat operational perspective, the 7.62 has application in CONUS urban situations as well, but the smaller 5.56, or 870 Police Magnum with TRUBALL 12gauge with flight control wad would be preferred due to over-penetration. I can do a lot more with 20 rounds of 7.62 Gold Medal Match than I could do with 30 rounds of M855 green tip 5.56. By the way, BETACO offers a 100 round beta c drum mag for the M1A for 7.62.