here I go again, blowing my patent chances...
...but I really don't mind, this is all mental exercise for me anyways.
Tungsten. Very heavy, somewhat brittle. Dense. And has a 6000 degree (give or take 192 degrees F) melting point.
Ok, so you make a sintered tungsten (the only way to do it), or perhaps ceramic piece. What is it? It's the external chamber, throat, and first 1" or so of rifling of the rife barrel. It's designed so that the rifling will thread over a hammerforging die with very small tolerances. The throat and rifling of this piece is where the rest of the barrel mates to it, so it's like a tube with a wavy surface not unlike the core-loct series of rod bolts for high-performance auto engines (it provides a locked fit without introducing pressure points).
So the chamber/lead insert is threaded onto the forging die's rifling, the barrel is slid over it, and beat into submission.. err, i mean hammerformed, onto the die. The rifling is now perfectly matched from insert to barrel, and the unit is irreversibly one piece.
Thermal expansion isn't really an issue... Tungsten simply doesn't do it much, meaning at *worst* we're talking about the steel barrel expanding and changing the tolerance of the rifling.
Why not the entire barrel? Well, for one tungsten is fairly brittle, and for the other.... let's say we have a .30 caliber barrel with an outside diameter of .80" at the reciever and .6" at the muzzle, 24" long, straight taper for ease of calculation.... it weighs roughly 5.3 pounds, all by itself.
Room temperature tensile strength of tungsten in the raw ranges form 100,000 to 500,000PSI. It has a Brinell hardness of 2570, and that's not a typo.
Other applications could be breech block inserts for blowback designs, recoil buffers, anything that requires extreme density. You could have a tiny little hammer on a pistol with the same inertia as a full size steel spur hammer....