Above a certain muzzle velocity, pure lead conicals need a copper gas check crimped onto the base of the bullet to prevent lead from being deposited upon the walls of the barrel.
This will occur for two reasons. First, smokeless burns hotter, and the faster you wish to propel the bullet, the higher the temperature behind the bullet will be.
Second, the faster the bullet travels down the bore, the more friction is created, thus the hotter the bullet becomes. This is where harder lead alloys come into play.
Ballistically, the base of the bullet steers the bullet, not the nose. Therefore, with a lead conical that travels fast, the base of the bullet must be protected.
At lower speeds, one can do this with a variety of wads. Wool, fiber, card, poly.
At higher speeds, a copper gas check is required, and the bullet mould's design must reflect this.
In a mould catalog you can often find the exact same bullet design, both with and without a gas check base.
Copper gas checks are crimped onto the base of the bullet during the lubing/sizing process.