As O'Kelly sees it, the ATF has been deliberately misinterpreting a key gun control regulation for decades because officials fear that following the letter of the law would allow criminals to build AR-15s and other firearms piece by piece with unregulated parts.
He said he voiced his concerns to an ATF official two decades ago, but was rebuffed.
Now, however, his view is gaining traction in courtrooms around the country.
In December, a federal judge in Ohio dismissed weapons-related charges against two men after O'Kelly testified that the AR-15 part at issue in their case was not subject to federal law or regulation.
US District Court Judge James G. Carr for the Northern District of Ohio called the ATF's long-standing interpretation of the regulation "unreasonable and legally unacceptable."
This key part, according to the Gun Control Act, was referred to as "the frame or receiver," which is, generally speaking, the body of a firearm in the area surrounding the trigger.
An accompanying federal regulation provided a precise, highly technical definition:
"That part of a firearm which provides housing for the hammer, bolt or breechblock, and firing mechanism, and which is usually threaded at its forward portion to receive the barrel."
The problem -- and this is where O'Kelly comes in -- is that he says roughly 60% of the guns in America do not have a single part that falls under that definition. The AR-15, for example, has a split receiver -- one upper and one lower. Neither meets the requirement on its own.
"For 50 years, ATF has been making this square peg fit in the round hole," O'Kelly told CNN, "when, in fact, it doesn't."