The inventor of a device to transform AR-15 rifles into illegal machine guns failed to persuade a federal appeals court that he was wrongly convicted in a Salt Lake City trial.

The U.S. 10th Circuit Court of Appeals at Denver ruled Monday that Scott Ray Bishop, 49, of Orem, was properly prevented from offering expert testimony about his converter kit, which he claimed was intended for educational purposes only.

As a result, Bishop will continue to serve a 33-month sentence at the federal prison in Florence, Colorado. He was sentenced in May 2018 on counts of manufacturing and selling machine guns, which have been illegal since 1986.

In an opinion written by Chief Circuit Judge Mary Beck Briscoe, the court said Bishop damaged his own case by not complying with federal criminal court rules of procedure.

Bishop represented himself in his January 2018 trial and failed to disclose before trial that he planned to offer expert testimony as the inventor of the AR-15 kits. Prosecutors objected in the trial, and U.S. District Judge Dee Benson barred the testimony.

Federal public defenders represented Bishop in his appeal. They argued Bishop’s constitutional right to present an adequate defense was violated by Benson.

But the circuit court said Benson still allowed Bishop to testify in his defense as a lay person and he was able to argue his innocent intent.

In the trial, Bishop said his kits, featuring a metal device that could be inserted into an AR-15’s triggering mechanism to allow multiple shots with one trigger pull, “was intended as an educational experience.”

“Some people chose to complete their kit in a way that I did not intend, and a choice that each of them had a right to make,” he said. “Not my choice.”

Bishop additionally objected that prosecutors presented inadmissible hearsay testimony against him — statements from people who bought the kits from Bishop and used them to convert their AR-15s to automatic fire.

The circuit court responded that expert testimony by a U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives special agent was sufficient to establish that Bishop’s kit allowed the manufacture of machine guns.

Agent Michael Powell explained that an AR-15 contains a part called a disconnector that prevents the rifle from firing a second shot without the shooter releasing the trigger and then pulling it a second time.

Bishop’s kit, Powell said, “overrides or negates the function of the disconnector while the trigger is pulled and the weapon is firing.” By disabling the disconnector, the kit allows an AR-15 to “fire automatically,” thereby meeting “the statutory definition of a machine gun,” he testified.

ATF agents investigated Bishop beginning in 2015 after learning of his online sales of the kits.

In 2016, an undercover ATF agent contacted Bishop through a Utah gun exchange and arranged to buy an AR-15 from him.

They met in a dirt parking lot off Interstate 15 and the agent surreptitiously recorded their conversation.

The agent asked if Bishop knew anything about “modifications” “like full auto stuff.”

Bishop responded, “I’ve got a few of them,” and “I don’t have a problem telling people I’ve got full auto.”

Bishop told the agent that “it’s not legal,” but insisted that “I’m not one that is concerned about that.”

It was not immediately clear Thursday whether Bishop will further appeal his conviction.

You can reach reporter Mark Shenefelt at mshenefelt@standard.net or 801 625-4224. Follow him on Twitter at @mshenefelt.