Laugh all you like, I still do not see 51 mentioned in the article ok? Perhaps you could show me where it is? If not, it could be, maybe, something different from what you are assuming....Btw, 7.62x35 carries a lot more punch than 5.56.
See my edit. They want commonality with their GPMGs which fire 7,62 NATO.
.300 AAC Blackout with a 125gr (supersonic) load at:
100yds 2036 fps with 1150ft/lbs energy (zeroed)
200yds 1828 fps with 927ft/lbs energy -7"
300yds 1635 fps with 592ft/lbs energy -24"
400yds 1460 fps with 592ft/lbs energy -54.7"
500yds 1308 fps with 474ft/lbs energy -101.5"
5,56 NATO M885 62gr load at:
100yds 2733 fps with 1028ft/lbs energy (zero)
200yds 2446 fps with 824ft/lbs energy -3"
300yds 2177 fps with 653ft/lbs energy -12"
400yds 1927 fps with 511ft/lbs energy -28.5"
500yds 1691 fps with 393ft/lbs energy -54"
7,62 NATO M118-LR 175gr load at:
100yds 2448 fps with 2329ft/lbs energy (zero)
200yds 2278 fps with 2015ft/lbs energy -4"
300yds 2114 fps with 1736ft/lbs energy -15"
400yds 1957 fps with 1489ft/lbs energy -34"
500yds 1807 fps with 1268ft/lbs energy -62"
600yds 1662 fps with 1073ft/lbs energy -100.6"
700yds 1522 fps with 800ft/lbs energy -151"
800yds 1387 fps with 748ft/lbs energy -218"
Now obviously if you were going to be shooting out past 3-400 yds you'd zero your rifle further out, but I'd say that gives you a good idea of why it would be 7,62 NATO and not .300 AAC Blackout. The only reason .300 Blackout might be chosen is not because it is a hard hitter at longer ranges (which it really isn't any way) but because of its subsonic capabilities with a supressor for close to moderate ranges (0-300yds).