El Reg gives us the skinny:
This brings to mind the Garand, and why it's chambered in .30-06. A 6.5mm round would probably been better - adequate stopping power combined with lighter weight. But the Army had millions of rounds in .30-06 in warehouses, and so the Garand was designed for that caliber.
But US military boffins at the famous Picatinny Arsenal have been working on this situation for some time. Since ammo weight and bulk is much of the problem, they have come up with a new kind of ammunition: Cased Telescoped cartridges.
In a cased telescoped round, the bullet is no longer attached to the tip of a brass case full of propellant powder. The new case is shorter, fatter and made of plastic, so weighing substantially less, and the bullet is sunk into the middle of the propellant which makes the whole round shorter - it has been "telescoped". A shorter round weighs less itself, and also means that the gun's action, feed equipment etc is smaller and thus lighter as well. It's a trick originally developed for tanks, to make the turret smaller and easier to protect.
According to the Picatinny scientists, their new LMG and a thousand rounds of its plastic-cased-telescoped ammo weigh no less than 20.4 pounds less than the current M249 (a version of which is also used by British troops) and a thousand ordinary 5.56mm brass cartridges. The new LMG shaves no less than 8.3 pounds off the 15.7-lb M249, coming in at just 7.4lb - actually lighter than a standard British SA80 assault rifle! This, perhaps, explains Specialist Smith's opinion that it would be reasonable for all soldiers to carry such weapons, rather than just heavy-weapons specialists.
Shifting from the poodle-shooter 5.56mm NATO round to either the plastic cased telescope round (or better yet to fully caseless - if they can make it work) would mean cheap ammo for all of us, but a big expense for the Army. And so technical committees will meet for years, tests will be run, and no decision will be made.
And Our Guys will have to hump heavier, less capable weapons into harm's way. Some may die because of it. Too much of the military works like this:
Two privates are on latrine detail, sweeping up soiled bits of toilet paper around the latrines. Just as they get all the bits swept up into a pile for collection, a gust of wind grabs one piece and sends it swirling into the camp, high above the heads of both soldiers. To their horror, it goes right in the window of the Colonel's office.Yeah, it's an old joke. I'm only laughing on the outside.
One soldier says to the other," I'll go in and get it, the old man is short sighted, half-deaf and naps a lot. I should be fine."
So off he goes, slipping quietly into the Colonel's office. He comes back a minute later, shaking his head.
"Well, did you get it?" says the first soldier.
"No" the second sighs," I was too late. The old man had already signed it."