The caliber and cartridge wars continue. What all agree upon is that 5.56x45 is anemic and insufficient for the current combat environments. This, of course, has been known for 15 years now, and yet our beloved Army has done nothing about the problem. Oh, they have brought out of storage the M14s that they saved from Clinton's attempted purge, but those were made over 50 years ago. Surely, there is something newer and better? Let us ignore NATO commitments for this flight of fancy. After all, it's not like NATO troops (other than some of the English speaking ones) are good for anything any more.
Serbia is in the process of rearming their special forces troops. They're replacing the 5.45mm AKs with newly designed 6.5mm Grendel AKs. These aren't your granddaddy's AKs, though. These are high quality steel and polymer builds with close tolerances, with decent barrels, using the little cartridge that could. Dust covers. Thumb reachable selector levers. Optics. Rails. These are high quality, modern weapons. I highly approve. Of, and of course, they have carbines, designated marksman rifles, and light machine guns. They appear to be using 108 grain bullets for the carbines, and 123 grain bullets for the rifles and LMGs. This makes a great deal of sense - it lightens the average rifleman's load a bit, and roughly equalizes the trajectories of the bullets from the different barrel lengths.
6.5mm bullets are not superior, but not by magic. They are superior because humans are firing them from human portable weapons. 6.5mm is the sweet spot for recoil versus bullet shape and weight. That means that 6.5mm bullets have relatively low recoil for a relatively high ballistic coefficient, yet still wide enough to be deadly to human sized targets. Don't misunderstand - .22 caliber bullets kill people - eventually. The target will bleed out over the next several hours without prompt medical treatment. (The US Army recommends shooting a target at least 5 times, at short range, to stop them with a .223. Not kill, stop. There's a difference.) But .223s don't cause shock outside of a very short range, especially with full metal jacketed military bullets. It takes a thicker, wider, heavier bullet to cause a large temporary wound cavity in the target. This is what pulps organs and ruptures blood vessels that aren't in the direct bath of the projectile. And that, my friends, is what causes an immediate cessation of hostilities by the target.
So, if we have a goal of using 6.5mm bullets, what cartridge should we use? The Grendel is an obvious choice, and probably the correct one for individual weapons. It has long legs, but not so much recoil as to make it unmanageable in close quarters. We could argue about using an AK or AR platform, but we Americans like our ARs, and there's no reason not to use a modified AR as the base platform. There's even a belt fed variant, that's a bit lighter than the m249 SAW.
What, then, shall we use for heavier weapons? The 7.62x51 is tried and true, if not optimal. Switching to a 6.5mm Creedmoor would be beneficial, but not dramatically so. Given that the 6.5 Grendal rounds have excellent performance at long ranges, combined with better than average penetration qualities, I think we could dispence with the 7.62mm NATO round all together, except perhaps for miniguns. Instead, for crew served, mounted weapons, we could use something heavier. After all, engagements at long range are relatively common now, and many adversaries wear body armor capable of defeating the .308 cartridge. The 7mm magnum is a fairly obvious choice, if we want to stay with a smaller caliber. The 7mm Nosler would have even longer legs. However, let us broaden our horizons.
In a crew served weapon, especially when mounted, recoil is not the primary objection. If it were, we would never use the venerable .50 BMG. So, we could invision a weapon using a cartridge of between .264 (6.5mm) and .338 (8.6mm) caliber. Since we intend for this cartridge to engage targets at long ranges and defeat modern body armor, the larger calibers would probably be better. So, we automatically go to the upper limit of .338, in either a Lapua or Nosler cartridge. These would give a medium machine gun long legs, without being quite as heavy as an m2a1 BMG. Size and weight of weapon and ammunition are still important considerations, of course. What other cartridges could fill the role? Most of the .30 caliber magnum cartridges would serve nearly as well. What we're really looking for is something that delivers over 3500 foot pounds of energy at the muzzle, with a high ballistic coefficient bullet travelling at over 2800 feet per second. What are your thoughts?