The old Brit L85 was the most advanced assault rifle in the world.
It actually got lighter as time went on, when the fiddly bits fell off. HK did some hard core glue work with it. The operating system is a fairly standard AR-18 knock off.
I like having the connection between the trigger and the sear as a light protected cable, with both ends under spring tension. That way there is minimal trigger play absorbed by the action bar flexing.
Before anyone gets too big a head, when the US M-1 Carbine was initially fielded, it had some problems: the magazine release was right next to the off safe button. Just when a soldier might decide he wants to use it, he would sometimes drop his magazine. Not the kind of thing to give you confidence.
Audi Murphy in his first combat patrol lined up his carbine on a German, fired and saw no effect. He blamed the carbine for the miss, and asserted that if he had been unable to blame the carbine, he could not have gone on to do the other things he did (normally with a Garand), but at least once with a .50 cal MG mounted to a Tank Destroyer.
the 5.56mmx45 is much more effective when the bullet hits meat at more than 2500 fps. (the effects decrease with decreased velocity but that is a useful threshold for discussion) The bullpup lets you have a long barrel, so that you start at higher velocity, so that magic number is maintained at longer ranges. That is why my patent: permitted a 28 inch rifle with a 24 inch barrel to get that high initial velocity with a short handy rifle. By contrast the M-4 only maintains that threshold velocity out to about 50 meters!
By contrast, the SAW (Minimi) has a plenty long barrel, but can engage the enemy at longer ranges, and at those longer ranges, the 2500 fps velocity may not be maintained.
The US 7.62 ball round has a thick bronze jacket, and normally doesn’t fragment. The German 7.62 ball has a thin steel jacket, and does fragment.
I don’t know what the effect of the shift from m855 ball to the new tungsten bullets will be. Tungsten is denser, but mixing in other metals to keep the volume up could help, or hinder.
Another factor is the yaw angle when it strikes the target. The bullet works best with a bit of yaw or pitch. If it hits exactly nose on, it tends to slide right through the target, merely stretching a bit of tissue along the way. If it has a bit of yaw or pitch as it strikes, the center of lift is about the 25% point, and it pitches more, turning sideways. The M855 or old M193 had a cannelure that would tear, and the bullet would fragment. The fragments compounded damage by cutting stretched tissue, leaving a baseball sized hole. That ‘cut while stretched’ was the idea behind the old Black Talon pistol round.
Of course the most lethal round in terms of terminal velocity would be a shotgun with number 1 shot. That is big enough to penetrate a man through and through, and small enough to have a bunch of projectiles. A 12 gauge 3 inch magnum puts out 25 each projectiles that are about 0.30 inch diameter. Like hitting a guy with a burst from a machine gun.
I like bullets in the 6.5-7mm range too, but if a 5.56 round can get equivalent lethality by increasing the velocity, well powder weighs less than lead! I think I like a 3000 fps 5.56 better than a 2600 fps 7mm round, and can carry more of them.