If the number is 10 or less it might be a valid investigative move.
What is wrong with the good, old-fashioned search warrant?
This sounds to me like a giant fishing expedition with lots of people's rights being trampled and little hope of success.
The source may be reliable, but all too often things can get blown out of proportion. In this case "all" may actually be "a few". I would want to see this from an official source before getting my panties in a bunch.
But no matter what, if I got a call from the FBI, or anyone else, for that matter, asking me to submit one or more of my guns for criminal tests, you better damn well believe I would be on the phone with one or more lawyers and a bunch of officials in my jurisdiction from the sheriff to the dogcatcher to make sure it was legit, and also to make sure that proper oversight and legal procedures were followed to the letter.
I am not the type to do anything like that quietly or without a solid paper trail.