The .50 was adopted with the purpose of being anti-material. It is often used in an anti-personnel role because it's got that extra reach. That would be where a .300 Win Mag or .338 Lapua Magnum would come in handy. It really doesn't have anything to do with weight so much as efficiency. The Barrett (SASR, M107, whatever) doesn't have the inherent accuracy needed to get reliable first round hits at those ranges.
You can fix that fouling problem pretty simply with a drop in piston kit...
The problem with the M14 is in it's complication. Granted, to a well trained warrior, it's not the worst, but in war-time environments, you need to keep things as simple as possible. That's the same reason the M110 isn't going to be the success everyone thinks it should. The bolt action rifle is simple and reliable. That's why it (and the 1911) are the only firearms that have seen over 100 years of military service.
