Just an idle thought; what if a semi-auto .50 was given an automatic sear and a "snail drum" magazine. SF troops would then have a "LHG" (for want of a better term) which would primaraly fire semi-auto at defined targets, but could be used to lay down supressive fire if needed (rear guard dealing with an enemy APC for example). Considering the size and weight of a "real" HMG, this would be far eaiser to transport and set up.
As far as the preference is concerned, I have never had the pleasure of firing a .50 rifle, and the only weapon of that description I have ever handled was a Boys anti-tank rifle (no, I havn't been in that long!), which struck me as being insanely large and awkward to fire. On a theoretical basis, I would think the Mac, with fewer moving parts and a bolt action would be more accurate than a Barret, which would have residual vibrations from the movement of the action to affect the barrel.