In Spooked (2009), Perverted (2009), and Savior (2010); Olivia's apartment number is shown as '4D'. In episode 12.8 Penetration it is shown as apartment '4E'.
When Stabler, Benson and Agent Dana Lewis confront the rapist he stands them off with a nail gun. The nail gun was completely harmless. They all have a safety device on the tip that must be compressed before it could fire a nail to prevent accidents. It could only be fired in direct contact with a solid object.