Pretty risky robbing a gun store without being armed yourself. Couple of bright ones here. OTOH, it was the ATF who "broke" up the plot. I don't trust those gun grabbing a**hats not to have set the whole thing up.
The BATFE didn't "break up the plot", nor even discover it. They were responding to information from the local sheriff's office. Guy had a shotgun cut down to a 16" barrel, 2" less than the max allowed with a special permit. That's why they were called in, the genetic defectives were (allegedly) violating the National Firearms Act.