Baker, there is some resistance as a bullet engages the lands. Lead bullets are soft and engrave with little resistance. Harder bullets resist the engraving to a greater degree. Jacketed bullets vary quite a bit in this regard due to jacket material and thickness and in the case of bullets like the A-Frame and Partition, construction. Monolithics like Barnes resist even more. That resistance to engraving or conforming to the lands tends to increase pressure at the very beginning of the burn and causes higher peak pressures.
