Dragonstriker

Materially it's one big weld (thinking of SLM, which is laser welding aluminium powder into the desired shape - in an inert atmosphere ofc). Not great for fatigue (e.g. you're very likely to end up with porosity), which might not be ideal for a receiver (I don't want to say it can't be done, but it'd need testing before saying it can be done). If this was really getting made then it'd be a much better option for the first half dozen prototypes, before the geometry was finalised enough for the big spend on the die for the extrusion.
 
Upper receivers are hollow bodies of almost constant cross section by definition (as they're enclosing the reciprocating moving parts), so it's a natural match for extrusion for full production

Yes if you want a truly decent bullpup, then you need to really engineer the thing for that at the start. I do have some ideas as to how best to do that, so maybe I will do that project someday.

My high pressure rifle cartridge studies have concluded, and, uh, I reinvented the 6mm Unified:
 

 
Performance is comparable in terms of operating pressure and energy delivered from a given barrel length. In this case, we're using it to send a 100gr BAT bullet at 3,000 ft/s from a 16.5" barrel. It out-performs all other rounds in consideration, while being smaller and lighter than the others, except the .224 (which is also an excellent performer, but which produced less energy. These subsequent studies intended to find a higher energy solution).
 
These studies have brought me a bit closer to the GPC camp, in an odd way. There isn't enough difference in terms of weight and recoil between something like this and a .224 Valkyrie equivalent to justify the logistics of two rounds. So then, maybe a unified high performance .224 caliber is the best answer (e.g., the .224 Archangel from my post on June 22nd), rather than two separate calibers. You sacrifice a little against this 6mm, but not much in the big scheme of things. And you're still out-performing rounds that are considered high performance even today. 

You could, if you wanted, pair this 6mm with some sort of ludicrously small high pressure round, something like a 4.65mm CETME or .204 Ruger on cocaine. I have my doubts, however, on the ability to make rounds like that in the kind of extreme industrial quantities required, while maintaining the quality control needed for consistent, low drag projectiles. If it came along with the opening of 3+ new ammunition factories, then yes that might be doable.

For the rifle squad? Belt feds of any kind are haram. Everyone should be armed with the same weapon chambered for whatever your standard rifle caliber is. So it is written. So it will be done.

Bullet doesn't have to be that heavy, but I'm a weirdo .224 cal crackpot. 80 grains would do it.
 
I'm a little lukewarm on 6.5 Creedmoor as a 7.62 replacement specifically because of the limited bullet volume, but it really depends exactly what you planned to use. I personally think US forces are arguably missing out by not having any small (<.50) caliber API, but opinions differ.

Yeah but let's be honest the GIGN ordering 7.62x39 806s is just IRL memes.