KETV (Omaha) reports:
U.S. attorneys had a conundrum on their hands -- they had the evidence to prove a convicted felon was in possession of a gun, but they couldn't prove the gun was a gun.
What [they] had on their hands was an American double-action revolver that was manufactured between 1880 and 1941.
The problem is that federal code states that the weapon is not a firearm unless it was manufactured after 1896....
I take it the reference is to 18 U.S.C. §§ 921-922, which ban felons from possessing firearms, but define "firearm" to exclude any "antique firearm," which is to say "any firearm ... manufactured in or before 1898." (The 1896 date appears to be an error in the news report.) So indeed the prosecutors didn't have the evidence to find the defendant guilty beyond a reasonable doubt of possessing a firearm manufactured after 1898, which is what the statute requires.
Fortunately for the prosecution, the felon-in-possession statute also bars felons from possessing ammunition, and the defendant was convicted of that.
Thanks to Sean Sirrine for the pointer.