This is the third half of the blog post on guns.
Interchangeability of Ammunition:
Ammunition is specifically designed. A .308 rifle cartridges will not fit in a 380 caliber pistol. One reason is the cartridge is too long. Another reason this doesn't work is the caliber is wrong; additionally, some cartridges are designed to produce a higher pressure when the round is fired. Some/Most guns can't handle these higher pressures of a different cartridge. Some can.
One of these is .38 Special cartridges in a revolver designed for the .357 magnum. These is an excellent article, explaining this interchangeablity, by Richard Malay at http://www.recguns.com/Sources/IIIB4.html
Rangy Lyman produced a chart showing the cartridges that are interchangeable. The chart is at http://yarchive.net/gun/ammo/cartridge_interchange.html
But what about the lack of interchangeablilty between the .308 and 7.62 NATO cartridge?
The .308 cartridge can be loaded for a higher presure than a 7.62 NATO round, so if you fire a 308 round in a gun designed to shoot 7.62 NATO, the rifle could be damaged.
Another danger is the 308 round is shorter then a 7.62 NATO cartridge. If you use a 308 cartridge in a rifle designed to shoot 7.62 NATO, the cartridge could rupture. A ruptured cartridge case would send very hot gases, from the burning powder, back into your eyes, face, and hands.
Chris Byrne, of The ArchAngel blog, has an excellent post on buying a scope for your rifles. His article is archived at http://anarchangel.blogspot.com/2008_08_01_archive.html You will have to scroll down to "Scoping Out." It is his August 4th post.