Lee Harvey Oswald/JFK ination question?

After reviewing over the ination I still have some questions to be answered...let's just say that Lee Harvey Oswald took part in it. How was he able to bring the gun into the book-depository without being unnoticed? Was it planted there? Wouldn't that point to a conspiracy? The other but more important thing i'm pondering is the 6th floor depository findings of three used bullet capsules, a hidden gun and boxes set up for the shot. If it is true that Oswald was able to reload fast and hit with pinpoint accuracy that would mean that he would have been a highly skilled marksman to complete that shot...So, my question is, why would a highly skilled marksman leave all that evidence at the crime scene? If he was able to bring in the gun, than he could have brought out the gun. Don't you think it's suspicious that a highly skilled marksman would leave bullet evidence and leave the station in perfect order? The common knowledgeable gun man would know that. What's your opinion?