The bullet that killed conservative commentator Charlie Kirk may not match the rifle used by suspected killer Tyler Robinson, a bombshell new court filing states.

Robinson, 22, is facing capital murder charges and a potential death sentence for Kirk's murder at Utah Valley University on September 10.

But his defense attorneys now argue that the Bureau of Alcohol, Tobacco, Firearms and Explosives 'was unable to identify the bullet recovered at autopsy to the rifle allegedly tied to Mr Robinson.'

The defense team may now offer the ATF firearm analyst's testimony as exculpatory evidence, they said in a motion filed on Friday to push the preliminary hearing back at least six months, Fox News reports. 

It also notes that DNA reports filed by the Federal Bureau of Investigation and ATF will take time for the defense team to analyze because reports indicated that several different DNA samples were found on some items of evidence.

'As these cases indicate, determining the number of contributors to a DNA mixture and determining whether the FBI and the ATF reliably applied validated and correct scientific procedures... is a complicated process which requires the assistance of various types of experts, including forensic biologists, geneticists, system engineers and statisticians, all of whom must review and evaluate' several different categories, the filing states, according to Deseret News. 

Robinson's attorneys added that they have received about 20,000 electronic audio files, videos and written documents that prosecutors have presented as evidence in the case.

'The defense team has devoted, and will continue to devote, significant resources, to processing discovery, including identifying materials not yet received to inform readiness for the preliminary hearing,' the filing states.

'However, the defense team is realistic and the comprehensive review required to determine what is missing will take hundreds of hours.'

'What is known at present is that Mr Robinson has not yet received the forensic case files and data necessary to investigate, through the use of qualified experts, the scientific reports the state intends to introduce at the preliminary hearing,' the attorneys continued.

Defense attorneys and prosecutors had previously met on March 12.

Based on that meeting, the defense said in the motion it believes prosecutors will introduce 'discrete "buckets" of evidence through three identified law enforcement witnesses at the preliminary hearing.