It occurred to me that I should probably pull some specific points into their own article, so I’m not repeating myself over and over. Let’s get on with it.
When choosing parts for one’s custom carbine, one ought to first conceive of the carbine’s purpose. What are you going to do with it? And why do the characteristics you plan to add help this? Admittedly, this requires some degree of maturity. It is not possible to do everything well. Purpose drives the build. Or at least it should.
Light for light’s own sake doesn’t make sense to me. Lightness is best for a gun that is carried much and shot a little. Competitors shoot their guns a lot, and the rest of the time the gun sits in a rack, a bag, or a cart. Even militaries optimize the gun for being actually used. The US Army went to a medium-profile barrel in the M4A1 for better shooting characteristics on full auto. Ditto the Marine Corps in the M27 (which also has an op-rod). And both services will load the rifle up with bipods, day optics, thermal optics, infrared lasers and, at least for special operations, suppressors. All of that is added weight, but all of that is added capability.
What of the historical exercise? Well, CONARC doesn’t exist anymore, and designing a weapon to fit in between the M1/M2 Carbine and the M14 rifle doesn’t make a whole lot of sense: neither are common service weapons of the US Army. Now, we could think about ‘can we make the infantryman’s carbine lighter,’ but then we should probably think about durability and reliability too. And some notion of acceptable accuracy. In other words, all of the things we’re going to do from before.
Those excessively nostalgic about some imagined past, like Pierre Sprey, conveniently ignore all of the innovations universally added to improve capabilities at the cost of weight. For Mr. Sprey, modern radars and computer systems make the F-16 able to do many more missions than the LWF was originally envisioned to do. Similarly, the M16/M4 has gained capabilities at the cost of weight. Everyone who can afford the added capability has gone for it.