Behold the HK G11 K2 caseless infantry rifle aka "Teutonic Space magic."
Plus assorted weird weapons.
Now than the American ACR and its son.
I could probably pull up more ACR entries online but I want to point to the G11 And Steyr ACR for a moment and there relation ship to the LSAT. That you also pointed to.
G11 was the closest any army came to adopting caseless ammo for small arms since the end of the rifled musket. It's ammo was used by LSAT to start development of its Caseless ammo.
And the Steyr ACR also Parallels the LSAT via it's method of operation ejection and that its ammo is polymer cased Teliscoped.
LSAT started based around one driver. Weight.
At the same time 2004 the XM8, had been driven by weight to but those who ran it attempted to make it a new rifle across the board without considering that the reason XM29 was broken. That it was it was to heavy. The objective weight was 6.81 kg loaded the real weight was 8.17 kg loaded. That 6.8kg is identical to the weight of the K11 a comparable weapon. But K11 had a bolt action grenade well XM29 was semiauto.
Breaking the two apart they hoped they could reduce the weight of them as they developed along the way. The XM25 however was upchambered for a 25mm round and farther had to be reinforced after a round detonated in the chamber. Adding more weight.
The XM8 did get some success in dropping the weight. A loaded XM8 weighted 7.5 pounds, that's identical to a loaded M4 carbine true, but the M4 only had it's irons at that weight where as XM8 had both irons and a red dot at that weight.
Still it wasn't the intended 5.7 pounds wanted, and the whole program was run wrong. And in 05 it was terminated.
So LSAT intended to do it right. The first thing though was that they changed there starting point. The problem wasn't the M4. As it stood then and now M4 really isn't much heavier than it's competition and is fine for now. (Also they had hoped to sinergize the ammo into the XM8)
The problem they argued was the Squad Automatic weapon. And the Army infantry school agreed meaning that had a real R&D program with the potential to lead to something.
The U.S. had a requirement for a new squad light machine gun back in 1984. The weapon they chose then and have used primarily since is the M249. A licenced variant of the FN Minimi.
Despite being called a "Light" machine gun M249 and the FN Minimi its based off weight in at 17 pounds empty that's almost as much as the loaded weight of the XM29.
And loaded with a belt M249 tipped the scales at 22 pounds. The lightest version I know of stripped it down to 13 pounds empty is the M249 SPW.
Now in US Navy Seals use at that time (1984 has since left service) was an obscure weapon called the Mk23.
This was a variant of the Cadillac gage/Stoner 63A
In the same 5.56x45mm with a weight of 11 pounds.
The Brand new (at the time) weapon FN Minimi for the Army 16 pounds
Stoner 63A LMG 11 pounds which would you rather foot match with?
Even as the Army adopted the M249 In Singapore the Ultimax 100 was entering service although that weapon had issues it was then and is still today a little over 10 pounds empty. And by 2004 Knights Armament company had and has today the rights to an improved version of the Stoner 63 The then KAC LMG at 10 pounds empty.
Still that's just the weapon then there is the ammo bringing it up to 22 pounds.
So LSAT focused in 04 on the LMG and ammo.
Trimming the weight of the LMG they could do they easily designed weapons to less than 10 pound weights.
But the ammo. Needed more science.
They chose two types. Polymer Cased Teliscoped and Caseless Teliscoped inspired by the ACR trials at that time they stuck with 5.56mm to save time and money.
The objective was at least a 40% reduction in ammo weight vs existing brass. Meaning a 100 round belt of ammo would weight 2 pounds not 3. 3 (with links) this would mean a LSAT LMG loaded with 150 round belt being around 13 pounds or less then the empty weight of the lightest M249 variant. You should be able to see the advantage at this point. Even compared to the loaded weight of a Mk23.