How does the author use foreshadowing in Intruder in the Dust?

In Chapter Three, Lucas' insistence that the bullet that killed Vinson Gowrie was not a Colt .41 round also provides a little foreshadowing. Lucas seems to know more than the reader or what he has already told Gavin Stevens, his lawyer. This leads the reader to wonder what kind of bullet did kill Vinson and how does Lucas know it is not a .41.