They also pestered him to saw off a shotgun many times. He refused and they just kept it up. Eventually he agreed and that's how ATF got the warrant for the raid.
The particular trick that the FBI played on him was that the shotgun in question was a break action. Randy finally modified the gun in a way he reasonably thought complied with the (probably unconstitutional) ATF requirement of a minimum 18” barrel, and 26” overall length. The “gotcha” was with the action open, and measuring in a straight line from the stock to the barrel, the gun was less than 26”.
It was a dirty trick and a bad-faith setup from the beginning.