ive seen a lot of lightweight AR15s on YT, some of which were 1.7kg. They used a fixed, thin-rod stock, and usually fiberglass or similar materials.

i'm trying to concieve of a weapon in 6ARC, with a weight of maybe 2.1kg.

would a rising chamber mechanism reduce weight, if so by how much....400grams?  

what about a rotating chamber, or a 'rammer' mechanism like the OTB-22

and lastly, how much weight does a typical M4-style stock create, are bullpups naturally lighter

Couple minutes of google foo and endless data on superlight weight ar15 are to be had,

https://docs.google.com/spreadsheets/d/1I6Qb6kAJjChEJ56qhznv2291c5UHRDyNXuSXGObumqM/edit?pli=1#gid=1203225381

https://www.tacticallink.com/lightweight-build-1.html

https://www.primaryarms.com/blog/Lightweight-Build-Giveaway

how much did the M1 carbine weigh?

could you make an M1 in 556 and still keep this weight, while still being durable for millitary?

5-5 1/2 lbs. weight. Nice guns, but a slight weight penalty means you can use a .300 Black Out AR, for a lower-cost, more reliable, and accurate weapon.

Johnson developed the 5.7 Spitfire, .30 carbine necked down to .224. 40-grain bullet 2700-2800, just needed a different barrel. For an M1 carbineish 5.56 NATO rifle, you would pick a Ruger Mini-14. Slightly lighter than an M4A1, BUT doesn't use STANAG mags, and is more expensive than an AR.

smg762 said:

how much did the M1 carbine weigh?

5.5 lbs.

smg762 said:

could you make an M1 in 556 and still keep this weight, while still being durable for millitary?

Possibly.  The Winchester LMR was essentially a slightly scaled-up M1 carbine, and reportedly weighed only 4.9 lbs.

Thanks Stan, forgot about the LMR.

Now bring that back in .350 or .400 Legend, Winchester would sell me a deer rifle!

do you know if a CT approach would reduce rifle weight, for example a 6.5mm rifle weighing only 2.5kg

Current technology can already build that rifle. The issue is, who wants to? In order to hit that weight while maintaining reliability you need to use more expensive materials and production techniques. Then you'll have a rifle that will have controllability issues at speed. Some of your troops will shoot it almost as fast and accurately as a 5.45/5.56; but most won't. 

And CT weight loss mostly comes from reducing the weight of the ammunition carried, not so much the weapon it's fired in.

At some point no material can substitute weight , for action to work reliably, for that you would need fundamental change to the action and ammunition , like casseless ammo

Other thing is at some point things also show in practical accuracy. I reckon with active FCS guns could be lighter and still shot with acceptable accuracy