Gun rights advocates are up in arms that a Texas gun dealer was sentenced to six months in prison for selling a firearm to an illegal immigrant, but a "middle-man" who bought the gun for the immigrant -- and who was in the U.S. illegally himself, but had a valid driver's license -- was never arrested, charged or deported in the case.

Paul Copeland, 56, a Vietnam veteran, was sentenced to prison time and two years probation in federal court last week for selling a gun to an undocumented alien, Hipolito Aviles, at the Texas Gun Show in Austin in January.

But Aviles wasn't the man who handed Copeland the money for the gun. That man was Leonel Huerta Sr., who presented as identification the valid Texas driver's license he had obtained before his visa expired in 2007.

Copeland claimed he was presented with a valid driver's license and had no way of knowing that the man he was selling the gun to was an illegal immigrant, or that he intended to give the gun to another illegal immigrant.

Huerta, in a phone interview with Copeland's attorney that was included in court documents, said he showed Copeland his driver's license in order to purchase the gun, and it wasn't until he was leaving the building that he handed the gun to Aviles.

But agents from the Bureau of Alcohol, Tobacco, Firearms and Explosives testified that Copeland should have known that Huerta was buying the gun for Aviles.

"Agents witnessed the Defendant (Copeland) negotiate a price for a handgun with Hipolito Aviles, who then handed cash to a second Hispanic male, who then handed Aviles' cash and his own identification to the defendant," the prosecution said in court documents.

"The Defendant in turn sold the firearm and handed it to the 'straw' purchaser, who then handed the firearm to Hipolito Aviles. The Defendant then instructed Mr. Aviles to hand the firearm back to the straw purchaser because he had 'bought' the firearm. Agents witnessed the straw purchaser hand the firearm back to Hipolito Aviles a short time later."

Aviles, the undocumented immigrant, was charged with possession of a firearm by a prohibited person, served a little over six months in prison and was deported on July 8, U.S. Attorney's Office spokesman Daryl Fields said.

But Huerta, the "middle man" who bought the gun for Aviles and also was in the country illegally, suffered no legal consequences in the case. He is believed to still be living in the country illegally.

"Instead of busting the illegal alien for buying, they bust the citizen for selling," said Paul Velte, founder of Peaceable Texans for Firearms Rights, a gun-owners rights advocacy group from Austin.

Copeland, in an interview with FoxNews.com, said he and several other witnesses, including Aviles and Huerta's son, who was present during the gun purchase, testified that the men never exchanged money in front of him. He said that throughout the purchase, he either talked with Huerta, who he says "spoke perfect English," or to the group in general, but never to Aviles directly.

"I never conversed with Hipolito at all," Copeland said. "The gentleman claims that he doesn't speak any English."

But one thing all sides agreed on is that Huerta, using a valid Texas driver's license, knowingly bought a gun for someone who was ineligible to buy one himself -- all the while knowing that he, too, was in the country illegally.

"SA [Special Agent] Jones interviewed Huerta, using an interpreter, and learned that he is also illegally in the United States and he showed SA Jones a work visa that expired in 2007," ATF wrote in its report of the incident. "SA Jones further interviewed Huerta and learned that he had just met Aviles for the first time at the gun show and he agreed to help Aviles purchase a gun because Aviles did not have proper identification."

But, unlike Aviles, Huerta was not arrested at the gun show that day. And though the report said he was "to be indicted," the ATF said he was never charged in the case.

"Apparently, Huerta was not charged because he was a witness," ATF spokeswoman Franceska Perot told FoxNews.com, adding that the bureau referred the case to U.S. Immigration and Customs Enforcement. ICE spokeswoman Gail Montenegro said there was no record of Huerta in ICE's system, indicating that there is no effort underway to deport him.

Perot said that, to her understanding, Huerta wasn't arrested at the gun show because agents there felt "he was just a go-between for Aviles." She said she couldn't say specifically why the decision was made not to prosecute him later, other than to say that situations like this happen all the time.

"It's a prosecutorial decision. They decide, are we going to get more time for this guy, for his part, or are we going to use him as a witness for someone else? So I guess that was their decision," she said.

Velte says that decision should "enrage all Americans."

"The illegal alien walks free, but the citizen gets convicted," he said. "The same government charged with controlling immigration is the one using illegal immigrants to attack its own citizens. Does this make any sense?"

When asked about Huerta, the U.S. Attorney's Office said, "we do not comment about our prosecutorial decision making process."

Attempts to contact Huerta were unsuccessful.

As for Copeland, Perot said he may not like the verdict in his case, but it was issued by a jury fair and square.

"He was found guilty by a jury, so if he feels like he was unfairly tried then there's an appeals process for that," she said.

Copeland says he intends to explore that process to the fullest.

"The appeal's being filed as we speak," he said.