Seating below the neck/shoulder junction can be bad for any bullet. Most FB bullets are about .0005 bigger at the base than the shank, and that can make a bullet fit the bore tight enough to make a difference.
How is the having the base of the bullet below the neck shoulder junction bad? And what difference does it make that the base of a FB buillet is 0.0005" larger than the shank (assuming that is necessarily so), since the whole thing is going down the bore anyway one way or another?
