The jump before the bullet contacts the leade – the tapered portion of the rifling where the engraving starts – makes a difference in the rate of pressure rise when the cartridge is fired. If you use a given COAL with a secant ogive bullet, and switch to a round-nose of the same weight – and use the same COAL – you’re in trouble with chamber pressure. You’ll most likely seat the bullet deeper in the case upon chambering, and cause a pressure excursion because there will be both no jump after firing, and a lower effective case capacity.

Also, I see no reference to the accuracy issue in what is written above. Usually, that is considered in light of a minimum jump between the case and the Leade, so that the bullet longitudinal axis remains coaxial with the barrel longitudinal axis, but with sufficient space to allow movement to begin to avoid overpressure from seating the bullet in contact with the Leade.

Finally, on definitions, SAAMI used to define Leade as: That section of the bore of a rifled gun barrel located immediately ahead of the chamber in which the rifling is conically removed to provide clearance for the seated bullet. Also called Throat or Ball Seat. That is, the tapered portion of the rifling at the beginning of the rifling.

Now the listed definition for Leade has been removed, and the issue is defined at Throat. Throat: The tapered portion of the bore of a barrel, immediately ahead of the chamber which is sized to provide clearance for the bullet of the loaded cartridge. Also referred to as Leade or Ball Seat and is associated with Free Bore.

That conflicts with your diagram above. It also defines Free Bore: A portion of the chamber, usually cylindrical, forward of the casemouth, of a diameter larger than the projectile, in which rifling is not present. See Bullet Jump. See Throat.

And finally, Bullet Jump: The distance that a bullet must travel from its position at rest in the cartridge case to its initial engagement of the rifling. Also Known As: Free Travel

The SAAMI definitions are somewhat of a mess, confessing that there is a jumbled understanding of what the terms mean. For decades, chamber meant the negative space into which the cartridge case fit, with free bore constituting the throat, and the Leade being the tapered portion of the rifling at the end of the throat that began the engraving of the bullet upon firing. The jump was the distance that the bullet traveled after firing before it first contacted the tapered lands of the Leade. Your diagram, which shows the bullet nose in contact with the lands, conflicts with both that understanding and the current SAAMI definitions. See: https://saami.org/saami-glossary/