Depends on the nature of the investigation. If the FFL is hiding gun sales, or selling under the table (Im not suggesting this was actually happening).Yup. It would verify actual physical purchases recorded in their financials. It's "reasonable", but if the facts are true... they weren't on the warrant so they had no legal authority to seize those records. It might have been an oversight, but if they've been investigating the FFL for more than 2 years, they undoubtedly knew they wanted those records and should have had them on the warrant.
Just guessing, but whatever judge signed off on the warrant may not have allowed it. If their probable cause was financial misconduct I can see where a judge might not want to sign off on 4473's... being out of the scope of the investigation since those records don't actually contain any financial information.
That doesn't mean they couldn't still push for them and maybe employ some intimidation tactics... which it appears they did... and it worked.
Also, I missed the part where we know what was on the warrant? How does that work? Im assuming a warrent means whatever evidence they can find. Note, I took the kids fishing today and not going back to read 5 pages at the moment cause im doing other stuff too.