Brian Vicente, one of the state’s most prominent medical-marijuana proponents, calls it a “travesty.”

Tony Fabian, one of the state’s foremost gun-rights activists, says it’s evidence of “hostility.”

What has forged this quirky convergence of advocacy — tokers, meet shooters — is a September letter from the federal Bureau of Alcohol, Tobacco, Firearms and Explosives saying it is illegal for medical-marijuana patients to own firearms.

Everybody who buys a gun must fill out ATF Form 4473, which asks: “Are you an unlawful user of, or addicted to, marijuana or any depressant, stimulant, narcotic drug, or any other controlled substance?”

Answer yes, and you don’t get the gun. Falsely answer no, and you’ve just committed a crime.

The ATF’s letter, sent out Sept. 21, clarifies that the bureau includes medical-marijuana patients in that group of prohibited buyers because their marijuana use is inherently illegal federally.

The letter also tells gun sellers that they could be in line for punishment if they knowingly sell to medical-marijuana patients.

“Any person,” bureau Assistant Director Arthur Herbert writes in the open letter to all gun sellers, “who uses or is addicted to marijuana, regardless of whether his or her State has passed legislation authorizing marijuana use for medicinal purposes, is … prohibited by Federal law from possessing firearms or ammunition.”

That rubs Vicente and Fabian the wrong way but in different manners.

Cannabis advocates see the letter as a further attack on state medical-marijuana programs, following a memo earlier this year from the Justice Department clarifying that the department views dispensaries as legitimate prosecution targets.

“From a patients’ rights perspective, I think this is a travesty,” said Vicente, the head of Sensible Colorado. “People shouldn’t be denied their constitutional rights based on their choice of medicine.”

Fabian, the president of the Colorado State Shooting Association, said he agrees with a state law making it illegal to possess a firearm while intoxicated — on any substance. But he said he is suspicious of the recent letter.

The Obama administration has said it does not consider it worth federal agents’ time to enforce federal marijuana laws on individual medical- marijuana patients.

Why, then, Fabian asks, has it decided to crack down specifically on gun ownership by state- law-abiding medical-marijuana patients?

“There is a hostility by both the ATF and the Obama administration toward the owners of guns,” Fabian said.

Other gun-rights advocates haven’t stepped forward to criticize something that medical-marijuana activists also oppose.

The National Rifle Association, which frequently butts heads with the ATF, has not put out a statement on the letter, and a spokesperson there did not return calls for comment. A spokesman for Rocky Mountain Gun Owners was also silent.

There don’t appear to be any cases in Colorado where people have been prosecuted for illegally owning guns because they are medical- marijuana patients.

Jeff Dorschner, a spokesman for the U.S. attorney’s office in Colorado, said the decision whether to prosecute someone would be made on a case-by-case basis.

John Ingold: 303-954-1068 or jingold@denverpost.com