[Unclenick]

The melting explanation is not possible for several reasons. One is the lower melting point of the copper jacket and the much lower melting point of the lead core would cause them to melt first, absorbing energy by enthalpy of the change of phase, acting, in effect, like ice cubes relative to the melting point of steel. Instead, you find broken jacket pieces on steel plates, clearly not melted to a splatter. Lead splatter does occur because of its extreme malleability, but I haven't seen it appear melted.

The other problem is that while there exist some ways to heat metal very fast, such as an induction furnace that heats it simultaneously inside and out, or stretching or deformation of the metal volume which produces internal stretching friction, there is no way to get heat out of steel in a flash. Nor is there any way to get it into the steel in a flash when it is applied at the surface. Both cooling and heating from the surface are limited in transient velocity by the simple specific heat and thermal conductivity of the steel. Period. A good example of that limitation in action is when a rifle throat develops heat stress cracks, which constitute the alligator skin pattern that appears when enough rounds have been fired through it. That pattern occurs because the heat of the burning powder and combustion gases only penetrate a very short distance into the surface while the bullet is in the barrel, expanding only the heated metal at the surface. The base underneath hasn’t had time to warm equally and the expansion differential between it and the hot surface produces the cracks. If the heat could enter the steel in a flash, that would not occur.

It seems to me we might want to distinguish a couple of different barrel bulging phenomena. People often fail to consider that propellant gas has mass and forward momentum. That gas is what produces rocket effect at the muzzle which can be responsible for 40% or so of total recoil in some high power overbore rifle loads. Relieving that pressure laterally before a bullet exits is why muzzle brakes reduce recoil even if they are jetting the gas in all directions simultaneously, and not just in opposition to recoil or muzzle flip. If a bullet is being chased by the mass of an expanding high pressure gas column and stops suddenly, the gas’s momentum piles it up forward and compresses it to much higher pressure. That creates a barrel bulge just aft of the stoppage. This is usually a very visible bulge and is typically spread over two or more barrel diameters, all the way up to, and including barrel bursting. I believe this is the phenomenon FL-Flinter is referring to as it explains the location of the bulges he describes.

The other phenomenon, which is the one I call ringing, is much more localized. It resembles ringing of a chamber by shooting overly warm cereal filler loads. It often is only a quarter of an inch or so in length, and its bulge is typically much shorter and less pronounced than you see in a gas pressure bulge. In a thick barrel you may not see the bulge at all with the naked eye. It is this ring that I believe to be an artifact of the bullet collision interface.

I’ve been PM’ing a bit with one of our European members who described four Anschutz .22 LR club gun barrels apparently suffering from the latter phenomenon. He could feel a cleaning patch slip past the ring, but could observe no bulge at all in the barrel exteriors. He had no micrometer with which to measure the barrel OD’s at the time. A micrometer would be expected to reveal at least a small surface upset. .22 LR barrels are usually fairly soft as barrel steel goes, but even so, the cartridge involved does not have the energy nor does it generate the volume of gas typically expected to produce a pressure bulge. Especially not very far down the barrel. It is, I still think, bullet collision collapsing the nose of the rear bullet laterally outward that produces a localized ring with small or no obvious external bulge. I am prepared to be proved wrong if it can be demonstrated that gas pressure can be localized to produce short rings. Interestingly, these small rings produced no observable accuracy deterioration.

You occasionally see one of those pictures of a sectioned revolver barrel that has four, five or six bullets piled up in it. It would be interesting to apply a micrometer to identify bulge centers in one of those.