ldholton,
Here's a post from a few years ago, posted by "dan oz," a Campfire member from Australia who knows a thing or two about metallurgy:
"Actually, bullets can indeed weld themselves to cases. It is a quite well-documented phenomenon, an example of diffusion bonding. Diffusion bonding has industrial applications too - I've done some work with these. You really just need two pieces of material of fairly similar composition (such as a gilding metal jacket and a cartridge brass case neck) in good intimate contact without anything between them (ie both nice and clean), a bit of time. Temperature also helps, but it is a solid-state process. Atoms essentially wander about, and end up crossing the boundary (if conditions are right) and taking up positions in the crystal lattice of the other piece. If enough atoms do this the boundary becomes sufficiently blurred to make quite a good bond. It is not a corrosion process."
