Hope this isn't a stupid question but: does bullet lube actually lubricated the barrel, or does it serve some other purpose?? And also why don't they lube copper jacketed bullets? Thanks in advance.

You'll probably get some better responses on the cast bullet forum, but bullet lubes for cast bullets provide two functions. One function is as a sealant, to prevent combustion gasses from traveling between the bullet and bore, which is one cause of leading. The other function is to prevent the friction between the bullet and bore from causing leading. The requirements of lubes to some extent varies with the bullet hardness, breach pressures, velocities and barrel lengths. Which is to say the best lubes in the world won't prevent leading in all cases, and in some cases you can get by with the worst of lubes, or even no lube at all.

The reason most copper jacketed bullets aren't lubed is that the rate of copper fouling is considered acceptable by most shooters. That said, the last few years have seen lubricants applied to jacketed bullets in the form of moly, lubalox, and Barnes blue coating.

Hard to improve on Paul's answer, but I have a little to add. Lead and lead alloys have a far greater tendency to gall (stick to another surface and tear off), than do copper and copper alloys. Lube helps prevent that. I ain't getting into the argument about how and why it does; all I need to know is that the good lubes do.

Copper and its alloys have a much lesser tendency to gall, some better than others. Lubing jacketed bullets has its drawbacks depending on the lube used and how it is applied. Most of the time, it is seen as not worth the trouble.

The idea of lubing jacketed bullets comes and goes in waves. It was quite popular among the high power competitors of the 20's and 30's. It fell out of favor as jackets and barrels got better and after some shooters ran into trouble because those lubed bullets also lubed the chambers and increased case head thrust dramatically. I hear that some of the moly shooters have run into the same thing, but at least they ain't shooting low number Springfields.

Another idea that came and went was lead bullets with a dry lube coating that amounted to a thin plastic jacket. I shot a few hundred of them back in the '80's and they worked pretty good in pistols. Haven't seen any lately and I wonder why they disappeared.