Thank you, Sturgeon, for your wonderful TFB article today.
Like you, I've come to view light infantry as a primarily weight-constrained force. In general, trading firepower for lighter weight is advantageous with respect to the currently over-burdened U.S. infantryman. As you acknowledge in the article, generating suppressive fires via DMR rather than LMG is a weight-efficient means of achieving a suppressive effect in many of circumstances. I, however, do not believe that a belt-fed weapon is necessarily a dead end in the AR/DM role within the squad. There is at least one belt-fed weapon that can achieve very good accuracy: the HK21.
The HK21, of course, achieves this by having much more in common with a battle rifle than a traditional LMG or GPMG, namely the closed bolt operating system, hammer, and fixed, top-mounted scope mount of the G3 from which it is derived. While the roller-delayed operating system may itself be a dead end, the HK21 provides a template that could inform future designs in that a belt-fed weapon can retain many of the advantages a traditional magazine-fed weapon if the magazine well is merely replaced by a belt-feed mechanism. The downside is that reloading the HK21 is arguably even slower and more cumbersome than a traditional LMG/GPMG. In the tear-down portion of that video, Mr. Vickers notes that one work around is use a starter tab. I, for one, would not want to be fumbling around trying to find the starter tab when my fine motor skills go out the window as rounds are impacting around me, so reloading without ANY belt handling is desirable. While H&K tried to develop a linkless feed system for the HK21, it didn't go anywhere. I'm dubious that a linkless feed system could be made light enough and sufficiently reliable for infantry use.
If one goal of a future M249 replacement is that it share ammo with squads ARs, I agree that something along the lines of the M27 makes a lot of sense. I also believe that a GPC is a dead end, and that a two-caliber system for the infantry is probably the way to go. Ideally, my proposal is that the military replace 5.56x45mm with a cartridge optimized for a vld-epr bullet in the 50-77 grain range (i.e., an optimized SCHV round) and replace 7.62x51mm with a cartridge optimized for a vld-epr bullet in the 90-120 grain range, all concepts that I believe Sturgeon, among others, has touched on over the years. Ideally a composite case having a traditional extractor groove would be used. To summarize Sturgeon's work, the 7.62 replacement is, in essence, a composite-cased .264 USA (or possibly closer to 6.5mm Creedmoor) firing a 6.5mm vld-epr bullet. Let's call it a medium caliber, high velocity (MCHV) round. While I do not advocate equipping all members of the squad with a MCHV weapon, I do believe that having one or two MCHV DMRs within the squad would be desirable and that replacing the M249s with these makes the most sense.
From a logistics point of view, it would be desirable to distribute all SCHV rounds in magazines and all MCHV rounds in belts, other than perhaps accurized MCHV loadings. For this reason, I think it would be worthwhile to investigate a conceptual successor to the HK21 as a SAW/DMR. One change that I advocate is moving to a gas-operated system with a fixed barrel, preferably a LW-profile barrel with a carbon fiber overwrap to increase rigidity, surface area, and thermal conductivity. The SCAR 17 with a lengthened upper receiver to accommodate a constant-recoil system might be a good starting point. The grunts would primarily use the weapon in semi-auto, but a limited full-auto capability would be available for engaging maneuvering infantry at a distance and in close ambushes. Reloading would still be an issue. This is where we borrow from the best SAW that never was, the XM248.
The XM248, among its many innovations, used a cam-driven sprocket to advance the belt. While WeaponsMan unfortunately passed away recently, his great discussion of the feed mechanism lives on. While the XM248 promised belt-handling-free reloading, the ammo boxes did have potentially fragile exposed "plastic grippers" that held the first round in the feed position. Additionally, there is the potential for misalignment of the belt and feed sprocket during reloading. While, I don't consider these to be deal breakers, we might do better in terms of reliability by integrating the feed sprocket with the ammo box. The ammo box would hold a round in the feed position via an anti-backup pawl, as in the XM248 design. While carrying around a feed sprocket in each "magazine" would add weight, I doubt there would be any penalty in terms of weight or bulk versus drum magazines, and it would enable truly care-free reloading. I imagine they'd actually be significantly less bulky than drum magazines and no heavier, if not slightly lighter, if a plastic belt is used. I propose 60-75 round drums as being standard. A backup, loose-belt adapter could be carried and inserted into the "magwell" if only loose belts for the GPMGs were available. The cam assembly, however, does prevent the use of a traditional hammer and trigger mechanism. We might get around this by using a linear hammer, as in the QBZ-97, or use the slightly more complicated cam system of the HK21.
What do y'all think?