A declaration from the Bureau of Alcohol, Tobacco, Firearms, and Explosives could leave millions of American gun owners in serious legal jeopardy.

In a letter made public on Tuesday, the agency informed a New Hampshire gun company that its AR-15 pistol model equipped with a specialized brace is actually an illegal short-barreled rifle and anyone who owns one could be subject to up to 10 years in prison or $100,000 in fines. Industry insiders put the number of AR-15 pistols equipped with similar specialized braces at about 3 million.

The decision appears to conflict with several similar determinations made by the ATF dating back to 2012, when the agency told Sig Sauer that the braces, designed to strap to a shooter’s forearm instead of being pressed against their shoulder, could be legally combined with guns that have barrels shorter than the 16-inch federal limit for rifles. The new letter calls into question the basic legality of the devices and leaves gun owners in flux just three weeks before the presidential election.

Carolyn Gwathmey, the public affairs officer for the ATF, said the letter