A boattail's base obviously "sees" more pressure than a flat-base, because there's more area to the base behind the full-diameter bearing surface of the bullet, and PSI is pressure per square inch.
But with a boattail much of the pressure is applied to the side of the truncated cone, not directly forward on the flat base. This difference is larger in high-BC boattails with very small flat bases and more elongated truncated cones.
Or at least that's the way I'd read it. Whether or not that's correct, the military studies documented quicker throat erosion with boattail bullets, after firing hundreds of thousands of rounds, and attributed it to gas blow-by.
