Clearly, we have a failure to communicate. (Its probably my fault, and I admit that; it's a problem)
Here is the fully nitty gritty.
1) A shell casing DOES NOT go down the barrel of the gun. It is discharged through the reciever out the side, or underneath the firearm. Thus, it does not contribute to erosive wear on the barrel.
2) Copper work hardens under physical stress. (Most metals do in fact)
http://en.wikipedia.org/wiki/Work_hardening Copper is the poster child for this, because it hardens so much. Physical stresses include compressive stresses in this case.
3) Gun barrels are made of alloys that favor being "flexible and soft". This is especially true of long rifle barrels, where there is increased stress from a longer chambered explosive force.
According to this source, the material used to make (at least this kind) pistol barrels is 415 stainless steel.
http://www.grantcunningham.com/blog_files/bb349783501752c295ed46d58226d889-994.htmlThe material datasheet for 415 stainless is readily available online:
http://www.metalcor.de/en/datenblatt/8/Note that it is formulated for "Toughness".
With metals, as this tool maker points out:
http://americancuttingedge.com/hardness-vs-toughness/As toughness increases, hardness goes down, and vise versa.
This means that gun barrels are not "Hard" metal, and as such, are usually much softer than other metals you will encounter. This is why a soft projectile is really necessary.
This is ESPECIALLY true in antique weapons, which have much more simplistic metallurgical compositions!
Now that we have covered the metal in the barrel, let's examine what happens with ballistic coefficients as the mass of the projectile drops.
http://en.wikipedia.org/wiki/Ballistic_coefficientBasically, a high mass projectile does not slow down much as it encounters air drag. It thus, does not need as much initial energy to retain its velocity as it travels.
Reducing the mass of the projectile greatly affects the ability of the projectile to reach distant targets, and greatly affects the accuracy of the projectile. In order to overcome those problems, the projectile must be delivered with significantly more energy. that energy is delivered by the powder charge in the shell casing.
The gun barrel must be able to withstand the increased energy yeild behind the lighter projectile. If it cant, it explodes.
That out of the way, here's what the medical library of utah has to say on the subject:
http://library.med.utah.edu/WebPath/TUTORIAL/GUNS/GUNBLST.htmlThe relevant section is quoted with emphasis:
Bullets fired from a rifle will have more energy than similar bullets fired from a handgun. More powder can also be used in rifle cartridges because the bullet chambers can be designed to withstand greater pressures (50,000 to 70,000 for rifles psi vs. 30,000 to 40,000 psi for handgun chamber). Higher pressures require a bigger gun with more recoil that is slower to load and generates more heat that produces more wear on the metal. It is difficult in practice to measure the forces within a gun barrel, but the one easily measured parameter is the velocity with which the bullet exits the barrel (muzzle velocity) and this measurement will be used in examples below. (Bruner et al, 2011)
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