During the final scene when Jules gives his take on his version of Ezekiel 25:17, he makes reference to "Mister Nine Millimeter." Problem is, that was a Springfield Arms .45... as it had been throughout the whole movie. It's highly unlikely that such a weapon could be re-chambered to fire 9mm rounds rather than .45ACP ammo, much less that someone would actually choose 9mm over the generally superior .45 slug.