How are shell casings left behind at officer Tippits' murder when Oswald was firing a revolver?

Maybe he dumped the shells (a stupid move for any assasin) but then again looking at the facts he had a .38 Special Smith and Weston Model 10 that he bought along with the Caracano that was used in the attempt on General Walkers life a few years before JFK assasination. The Model 10 holds 6 rounds in the cylinder, he fired four. So that's saying that he hand ejected four shells left 2 live rounds in and started to reload. To keep in mind Lee H. Oswald was an ex Marine, so he would have been trained to keep a fully loaded sidearm, maybe as he remembered his training started to reload fearing that that other officers were near by and another firefight would insue, as he was reloading perhaps he panicked and ran ( there are reports of a man running at top pin with a revolver in hand) he was found with the weapon in his possesion. A funny fact he was a terrible shot in the Marines, he missed the General from 100 yards.