There's a new twist in the case surrounding the assassination of conservative activist Charlie Kirk ... new legal documents claim the bullet used to commit the crime does not match the rifle tied to suspect Tyler Robinson, TMZ has learned. According to the legal docs obtained by TMZ, Robinson's defense team argues there's a discrepancy between the ballistic evidence and the weapon prosecutors say is connected to Robinson ... who's facing multiple charges -- including aggravated murder and felony discharge of a firearm causing serious bodily injury -- in Charlie's slaying. Robinson's attorney claims the Bureau of Alcohol, Tobacco, Firearms and Explosives conducted a summary report which indicates the ATF was "unable to identify the bullet recovered at autopsy to the rifle allegedly tied to