Before Mel shoots Rony he cocked the Hammer of his revolver. When he shoots Ronny one can clearly see the hammer fall. In the next shot the hammer is cocked again and Mel decocks it. Revolvers do not cock themselves after firing a shot, unlike semi-automatic pistols.
After Ronny shoots the cop in the head, he then steals and wears his uniform. The uniform should be a bloody mess, instead of completely clean and pristine and the uniform should be too small and tight on the heavyset Ronny, because the cop originally wearing the uniform is of normal build.