Originally Posted by rschlos
I just read a bit online about this, .451 for jacketed tips and .452 for lead tips, either way I would like to hear some more on this subject.
Go to the forum "CastBoolits" and learn more than you want to know.
"Boolits" is what some people have decided is the correct spelling for bullets made of cast lead, reserving the term "Bullets" for jacketed projectiles.
Lead is softer than copper (the most common jacketing material). But it melts when exposed to the hot products of combustion (the gasses blowing by the sides of a bullet that does not fill the bore and seal it).
If the hot gasses can get past the base of the lead bullet, the lead tends to melt a little, and inevitably deposit on the inside of your barrel's bore. "Leading" is the term and to be avoided if possible.
So, if the lead bullet is just a little oversized for your bore, it will get squeezed down to an exact fit. This squeezing is called "swaging".
If you think this is complicated, try shooting a revolver, where the bullet passes through the chamber throat, past the cylinder/barrel gap and enters a forcing cone before encountering the barrel proper, and the rifling.
Anyhow, that exact fit seals the bore. It is hard to get a bullet to bump UP in size (It is doable, and called "obturation", or "upset", look it up with google or the search feature.)
So that is why lead bullets should be slightly larger than the diameter of your bore (measured at the grooves, not the lands).
Copper-jacketed bullets are harder to swage to fit, thus must be closer in size to your bore. But the copper fouling is lighter in amount less problematic to shooting than leadin(harder to clean out, though not needed as often).
Get a copy of ABC's of reloading or most any loading manual that talks about fouling of barrels and most of your questions will be answered. I hope this little intro serves to whet your appetite for more knowledge.
The lay terms "tip" or "head" for bullets is common enough, but ballisticians use specific terms. The most egregious mis-use is to call an assembled cartridge (consisting of a primer, propellant, bullet and case) a "bullet". Precision of terminology (especially in print, where nuances are often lost and feedback is slow) is vital to communication. Plus, some people get uptight. Nobody in this thread so far (just a little teasing), but sometimes.
You want a lead bullet to be guaranteed to swage down to seal the bore to prevent leading and get good accuracy. Bullet manufacturers cannot depend on all bores being exactly the same size, so oversizing (just a little) is standard with lead.
You want a jacketed bullet to be closer to the actual bore size because it does not swage down as easily as lead (and you don't want to increase pressures unduly). Fortunately, copper does not melt and foul your bore like lead does.
Good luck and thanks for asking our advice