Many gunsmiths now have bore scopes that allow the inside of any barrel to be inspected. That, not a chamber cast, is the best way to determine the amont of throat erosion and general barrel wear.
Just FWIW, for anyone unaware of or confused by the term "throat erosion", here is what it is and the reason.
When a rifle (handguns too, but we are talking rifle here) is fired, the powder starts to burn rapidly at high temperature, and pressure rises rapidly. One of the first things that pressure does is expand the case neck. This is important; if the chamber neck is too small or the brass too thick to allow that expansion, pressures go sky high.
So the case neck expands, but the bullet's own inertia keeps it from moving for just an instant. In that instant, hot gas and burning powder particles rush past the bullet and into the barrel at high speed. Now any gas passing through a small gap speeds up, just like a stream of water speeds up as you tighten down the nozzle of a garden hose.
Someone compared that stream of hot gas to turning a welding torch into the barrel throat, but it is actually worse than that, since the gas and burning particles don't just heat the barrel, they "scour" the barrel throat like a sand blaster. This is "throat erosion." Once the bullet begins to move, it closes off the gap between itself and the barrel, the gas is slowed by the bullet, and the scouring ends.
Now, as the throat erosion continues, it scours the throat farther and farther out, and the "leade" where the bullet is smaller than the bore, is pushed further and further out as well as eaten away. Eventually, a point is reached where the gases will cool enough that the scouring stops and the damage will go no further.
But what is left is a part of the bore which is larger than the bullet, and long enough (sometimes as much as 5 inches in machineguns) to allow the bullet to twist and skid. By the time good rifling is reached, the bullet is already damaged and accuracy is destroyed.
Is there a way to prevent throat erosion? No. Obviously, a light powder charge will cause less erosion, and powders* notorious for causing it can be avoided. Slow firing, allowing the barrel to cool between shots, will help reduce erosion and extend barrel life.
As implied above, things are at their worst in machine guns with very rapid firing. Machine gun barrels erode very rapidly unless cooled or some resistant material (Stellite) is used to line the barrel. But even then, erosion is only slowed, not stopped.
*Double based powders contain nitroglycerin, which causes more erosion but they have other desireable properties so they are still used. One of the worst powders for causing erosion was the old British Cordite, which had a high nitroglycerin content.
Jim
