HELENA, Mont. – Robbie Regennitter is a registered medical-marijuana patient.

Regennitter says he ingests approximately 10-20 milligrams of THC — the active compound in marijuana — each night before bed to ease the painful symptoms of gastroesophageal reflux disease and an esophagus condition.

Regennitter is also a hunter and gun owner. According to a new memo from the federal Bureau of Alcohol, Tobacco, Firearms and Explosives, it is illegal for him or any registered medical-marijuana patient to own or possess firearms or ammunition.

The letter written last week by ATF Assistant Director Arthur Herbert to all federal firearms licensees gave them guidance on what to do if a firearms customer reveals that he or she is a medical-marijuana patient.

According to the letter, "any person who uses or is addicted to marijuana, regardless of whether his or her state has passed legislation authorizing marijuana use for medicinal purposes, is an unlawful user of or addicted to a controlled substance, and is prohibited by federal law from possessing firearms or ammunition."

Regennitter said he was stunned when he learned about the memo.

"At the moment it concerns me, but I'm not going to stop taking medical marijuana, and I'm not going to give up my firearms," Regennitter said. "I don't use (THC) recreationally. I use it because it helps me."

Jon Svaren, a 15-year Navy veteran who was honorably discharged in 2009, is a medical-marijuana patient who is recovering from a surgery last November to repair a severe injury to his back.

Svaren is also a gun owner who hunts and uses guns on the farm to control vermin.

"To take away my Second Amendment rights is contrary to everything I've ever fought for and contrary to every oath of enlistment I've taken," Svaren said.

Gun rights and medical-marijuana advocates both expressed outrage over the letter, which they say singles out a specific group of citizens and attempts to strip them of their Second Amendment rights.

"The cannabis issue has become representative of nationwide concerns," said Kate Cholewa, a board member of the Montana Cannabis Industry Association. "Citizens are increasingly concerned that the government, rather than expressing the will of the citizens, now sees itself as separate from the citizens and is imposing their will upon the people."

Sixteen states and the District of Columbia have passed laws legalizing marijuana for certain medical conditions, but the federal government classifies the drug as a schedule 1 controlled substance and thus illegal for any use.

According to ATF spokesman Drew Wade, the Herbert letter was intended to provide guidance to federally licensed firearms dealers in complying with federal firearms laws and was not intended to speak to consumers of medical marijuana.

"We received a number of questions from federal firearms licensees and gun dealers on (medical-marijuana patients), and we felt we needed to provide some clarity so they can be in compliance with the laws," Wade said.

Officials for the National Rifle Association did not return calls seeking comment, and Larry Pratt, executive director for Gun Owners of America, declined to comment.

"I can tell you why a lot of organizations won't talk about it: Marijuana is a lightning-rod subject," said Dave Workman, senior editor at Gun Week, a twice-monthly newspaper that covers legislative and regulatory issues related to guns.

"The media — and the gun prohibitionist lobby in particular — would say the gun lobby wants to arm drug addicts," he said.

Gary Marbut, president of the Montana Shooting Sports Association, did weigh in, saying his organization believes "it is more than unfortunate when a constitutional right, the right to bear arms, that people have reserved to themselves from government interference, is arbitrarily taken away by what many see as an overbearing and overintrusive federal government."