A Utah man imprisoned for transforming AR-15 rifles into machine guns wants a new trial to further his claim that he thought his conversion devices were akin to legal bump stocks.

Scott Ray Bishop, 48, of Orem, is serving a 33-month federal prison term in Colorado on felony charges of unlawfully engaging in the business of manufacturing machine guns and illegal possession and transfer of machine guns.

RELATED: Feds: Utah man illegally made machine-gun converters for AR-15 rifles

A Salt Lake City U.S. District Court jury convicted him in January and Judge Dee Benson sentenced him May 24. He was charged in January 2017, booked into the Weber County Jail and soon released pending trial.

In appeal documents filed at the U.S. 10th Circuit Court of Appeals in Denver, Bishop’s lawyers assert that Benson improperly blocked Bishop from testifying during the trial.

“As the designer, manufacturer, and ultimate seller of the device at issue, Mr. Bishop should have been allowed to testify about what he believed the device he designed was and what he intended it to do,” said a document filed by federal public defenders.

Bishop represented himself in the trial and repeatedly drew admonishments from Benson for not following proper court procedures.

Benson, according to the court record, told Bishop, “... here you have, as I understand it, an explanation that sounds like what you’re saying is that you created a bump stock type device, which I assume the ATF knows is legal.”

A U.S. Bureau of Alcohol, Tobacco and Firearms and Explosives agent testified in the trial that Bishop’s conversion device turned semi-automatic AR-15 rifles into illegal automatic machine guns.

Bump stocks, which fit over the stock and pistol grip of a semi-automatic rifle and allow the weapon to fire hundreds of rounds per minute, mimicking a fully automatic firearm, were used by the Las Vegas mass shooter.

Bump stocks were considered to be legal because they didn’t convert a semi-automatic firearm into one that is fully automatic, able to fire multiple rounds with a single trigger pull.

Bishop’s appeal attorneys said prosecutors were required to prove Bishop’s converted firearms were machine guns, but Benson barred him from testifying because he had not previously discosed his defense theory to prosecutors and the ATF.

“You have converted this trial into a trial by ambush,” Benson told Bishop, adding that allowing him to testify on the function of his devices would constitute expert testimony, which is always vetted in court before trial.

But his attorneys argued in appeal documents, “The definition of ‘machine gun’ encompasses not just a firearm, but also the mental state of someone designing parts for firearms. Mr. Bishop’s design of and intent for his device, or the ‘machine gun conversion kit,’ is at the center of the case and his appeal.”

Because Benson would not let him testify, Bishop’s Sixth Amendment right to mount a sufficient defense was violated, the appeal said.

The attorneys have filed a motion urging the Salt Lake court to release Bishop from prison pending the circuit court appeal. The U.S. Attorney’s Office is opposing the motion.

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