Yes, I am familiar with the m249. However, you are comparing a weapon that was designed in the 80s, with a weapon that was designed as if it was 100 or 200 years ahead of our current time. In this case I feel I must quote Arthur C. Clarke "any sufficiently advanced technology is indistinguishable from magic". However I would argue that the weapons in destiny are not all that advanced. Lets look at the image you posted of the Tamerlane shall we. I believe the loading mechanism might not be too dissimilar from what we see in the M2 Browning Machine Gun, where an arm rides up to the feed tray and pulls the round down on the face of the bolt. In fact looking at the Tamerlane - B there seems to be more than enough space in the sloped area just behind the feed tray for just such a mechanism. If it doesn't like like an M249, then chances are it doesn't operate like an M249.