There is a big difference between dumping a spare barrel and choosing a marginal weapon that may save your life to avoid humping a couple of pounds. SLA Marshall's somewhat believable books chronicle the Army personnel dumping everything, spare ammunition, grenades, over the matter of weight. I would not expect the same behavior from Marines. HMGs were rarely carried. I'll give you another theoretical instance that you can pick apart: Suppose your assistant gunner and ammo bearers are humping cans of belted ammunition in addition to a spare barrel in a piece of web gear that may be a genuine hassle to keep on board comfortably. You ask the SGT if it's ok to ditch the barrel and pick it up later or just forget about it. If there has been no use of it for several days, it might be a great idea to dump it in favor of bullets. Gas masks are dumped by the hundreds, was there a "lesson learned" on that?