South Dakota will support Montana in a federal lawsuit asserting states' rights to regulate guns that are manufactured and sold within a state's borders.
Montana was the first state in the nation to pass a "firearms freedom" law. The law claims that as long as the guns aren't involved in interstate commerce, the federal government should have no say over them.
But the federal Bureau of Alcohol, Tobacco, Firearms and Explosives is not going along with the law, writing to all licensed gun dealers in Montana last year telling them they were still bound by federal gun regulations.
Pro gun groups sued, and a trial is scheduled to take place in December to resolve the dispute.
South Dakota passed a similar law this year, and will join up with Montana in its battle.
"We joined because we can do it very cost-effectively, there is the impact directly to South Dakota because of our (firearms) manufacturers, and because we have a statute now in law that is very similar and could receive a very similar challenge," Attorney General Marty Jackley said.
Attorneys general in Wyoming and Utah, which have passed similar laws, also are filing briefs supporting Montana.
"This is our ability to say to the Montana court that this is an important issue more than just in Montana," Jackley said. "We have similar legislation in South Dakota, we have similar concerns about the expansion of the federal government into states' rights, and therefore we want you to take a close look at this."