Roy,
My thinking is that yes, the bolt does "slide" down the face of the receiver upon firing. The lever holds the bolt up, resisting this motion. I suspect that the bolt actually bends down very slightly where the lever contacts it, making it "shorter". If that is true, then the receiver bends up, also very slightly, getting "longer". All of this happens because of that angle where they meet. During the few milliseconds when the pressure is high, the cartridge stretches to fill the gap. When the pressure drops, everything goes back to its original position, making the whole assembly an interference fit. It's a lot easier to bend steel than to compress it.
Just my 2 cents.
