1. Why is it really all that important to retain the AR-15 platform? We've officially changed the service rifle three times in the last century. Even though I really don't see any incredible leaps of performance above the AR platforms, save perhaps the idea of the gas/piston systems, it shouldn't really be all that difficult to switch.
The main real-world argument is the billion dollar price tag for fielding a replacement.
The main argument in a hypothetical situation is perhaps not that we necessarily need to keep the AR-15 pattern rifle, but that any replacement needs to incorporate the strengths of that rifle, like its modularity and superb control layout. A replacement may bring additional strengths to the table, like a gas piston or whatever, but it needs to keep the existing strengths or build on them.
Not that I'm the first person to have this idea. The SCAR, XCR, and Masada all seem to be built with this basic concept in mind.
2. Why, if we are going to retain the AR-15 platform as our basis, is it necessary to retain the same receiver length? Even if we didn't step up to the AR-10 platform as a basis, why couldn't we just make an AR-style rifle system with an intermediate action length that would accommodate something in-between the 45mm and 51mm cases (say, 48mm?)
The modularity of the AR-15 pattern revolves around the lower receiver. Since the lower receiver includes the mag well, it imposes the main limitation on that modularity.
You could build a lower sized for 7x46 or some other hypothetical round that exceeds 5.56mm OAL, but then you're talking about an entirely new weapon -- like an AR-10, not like a modified AR-15, essentially.
Which is not to say the idea does not have merit, but it's understandable why there's less interest in that than a drop in mod that works on a standard AR-15 lower. Both on the industry side as well as the agency side, it's driven by economics rather than ideal ballistics, but "good enough" has always been the enemy of "best."
3. Although the 6.5 Grendel seems on the face of it to have superior ballistics to the 6.8mm Remington SPC, there seems to be more support for the SPC. Why is that? Other than the fact that the Grendel is a bit ungainly looking, are there significant drawbacks to it?
Basically? Grendel is a target round, 6.8 Rem SPC is a combat round.
Grendel has great ballistics (using pricy bullets, which a military load likely would not) out past 400 meters. Combat doesn't happen there. And it gets them by having a case geometry that sacrifices feeding reliability in favor of high BC bullets. Even a 1% reduction in reliable feeding to get a round that works great at 600 meters is, pretty much, answering all the questions an service round needs to answer . . . wrong.
Rem SPC is geared towards 400 meters and in, where combat does happen. It's optimized to feed reliably in a semi auto or automatic weapon. It's limitations are primarily those forced on it by the AR magwell dimensions, but it's hard to say if this is a bad thing. Ballistics might be better if it was slinging a 130 to 140 grain projectile, but recoil would be increased as well.
4. Judging from the muzzle energy of the .223 Remington (~1300 ft-lb), 6.8 SPC (1750 ft-lb), and 6.5 Grendel (1900 ft-lb) [all from 24 in barrels], it would seem that we should be looking for a round that would produce something in the area of 2000-2400 ft-lbs ME for superior performance in everything from a carbine to an SAW. Why wouldn't we just go with the .243 Winchester, .260 Remington, or 7mm-08 Remington (or a 48mm cased version of any of these)? Any of these would be lighter than the .308 Winchester/7.62x51, which has always been cited as one of the major advantages of the 5.56mm rounds v. the 7.62mm rounds, and should exhibit less recoil (other commonly cited advantage).
Bulk and weight to begin with. Like GunTech said, 7-08 is pretty much just like 7.62x51 in terms of weight and in terms of the bulk it takes up in magazines, etc. Keep in mind that when you're loading up a fighting load on say a humvee or in a Bradley, you get basically double the amount of ammunition for space and weight with 5.56mm that you get with 7.62x51. With anything based on .308 but necked down, etc., you're still getting that halved amount, maybe with some weight savings, but still taking up the same amount of space in the truck/track/whatever. It ain't much different when you look at how many mags a guy can carry on his body armor or LBE or whatever.
What we're looking for is something probably in the 1500-2000 ft-lb range. Most of the guys carrying are going to be carrying a rifle or carbine, not a SAW, and I personally don't think we should be talking about sacrificing rifle/carbine performance to plus up a SAW. A SAW is a bullet hose for squad level suppressive fire, it isn't a medium machinegun, and it should not be built as such if it means taking performance away from the rifle/carbine.
And remember when talking about rifle/carbine performance, we're not just talking about foot-pounds at the muzzle. The weapon's speed of handling, speed back onto target for follow ups and onto additional targets, is as or more important as long as you're getting acceptable thump for lethality. The higher the bullet weight and velocity, the more recoil you get and the less suitable a weapon you've got for real-world combat marksmanship (even if it causes stirs at Camp Perry, etc.). What is ideal for a service rifle/carbine cartridge is just enough lethality to get the job done (how much is "just enough" is the question), but light enough that I can be driving it onto the next guy who needs killing while the first one is still headed to the floor.
.