There are several federal excise taxes on firearms. The main ones are (dispensing with the fine definitional details and Internal Revenue Code citations):
A tax on transfers of firearms of $5 per concealable firearm and $200 per firearm on certain other firearms;
a $500 per year firearm's dealers tax which is increased to $1000 if you import more than a certain number of firearms per year;
a 10% of sales price excise tax on pistols and revolvers; and
an 11% of sales price excise tax on other firearms and on ammunition.
Of course, like any other business, firearm's dealers also have to file annual income tax returns on their revenues less their expenses on tax forms that depend upon the form of organization of the business (e.g. C-corporation, S-corporation, partnership/LLC, trust, etc.).
The IRS has the right to examine the books and records of people who owe or are believed to owe taxes without going to a court to obtain permission to do so. Treas. Reg. § 1.601.105.
Subject to certain exceptions, information obtained by the IRS in a review of a taxpayers records and their return information are confidential. 26 U.S.C. § 6103.
There are criminal penalties for (slightly over simplifying) tax offenses including willfully failing to keep records necessary to file tax returns, willfully attempting to evade or defeat taxes, willfully failing to collect and pay taxes where required by law to do so, and willfully making false statements to IRS employees or on returns. 26 U.S.C. § 7201-7212.
If the government provides probable cause to a federal judge or magistrate in connection with a possible criminal tax prosecution that there is probable cause to believe that a taxpayer has committed a criminal tax offense, then the government may obtain a search warrant to seize records without the advanced notice available in the usual civil record examination process.
For example (not based on any facts I have been told about or read about in this particular case), suppose that a former employee of a firearm's dealer or an ex-spouse of one of the owners of the business, told the IRS that the dealer intentionally lied on their excise tax return by underreporting the number of firearms the dealer sold and that the dealer then kept the sales taxes collected from unreported buyers who completed Form 4473, and told the IRS criminal division investigators where the Form 4473s were kept at the dealer's offices. In that case, it would be routine for the IRS to obtain a federal search warrant to obtain those forms to seize and review in connection with an excise tax fraud investigation.
If there was just an income tax audit, bank records and accounting records obtained by subpoena from banks and accountants would usually be sufficient. But this would cease to be the case if the inventory records obtained by subpoena from the dealer's suppliers, for example, and the accounting records, didn't match.
If there was evidence of unreported firearm sales income, or of unreported excise taxable sales, the IRS would usually need to compare the the Form 4473s of the business to its income and excise tax returns filed with the IRS. It would be routine to obtain these records with a search warrant rather than a civil office record review with advanced notice if tax fraud was suspected.
The Form 4473s due to IRS confidentiality requirements, wouldn't be publicly available to anyone by the IRS and criminal investigators (not even members of Congress or local law enforcement), and would probably only be presented in redacted or summary forms in a criminal tax fraud prosecution at trial.
In all likelihood, the IRS doesn't care about the customers who filled out the Form 4473s at all, and isn't even bothering to investigate them (except possibly to spot check for fake social security numbers or names, which appear on the forms, to see if fake information was used to Form 4473s used to substantiate tax records).
Instead, the IRS is probably simply tallying them up and noting dates of sale, putting them in a spreadsheet, and seeing if they are different from what was reported to the IRS on the dealer's tax returns.
If the dealer, for example, paid the proper excise taxes on 800 guns in 2022, but had 1200 Form 4473s in banker's boxes at its offices, then the people involved in the tax fraud at the dealership are probably going to go to federal prison for a few years. Indeed, they would probably just plead guilty rather than going through a futile trial where documents with their own signatures on them from boxes seized in their shop clearly establish their guilt in that case.
Federal firearm excise tax fraud prosecutions aren't terribly common, but they are about as plain vanilla as they come in the world of federal criminal prosecutions. This certainly doesn't portend any threat the Second Amendment rights or any crack down on the firearms industry.
In terms of the message this sends, this is really no different that seizing the electronic records of a gas station that filed false gasoline excise tax returns to show how much gasoline was actually sold, and prosecuting the people who engaged in the tax fraud for that.
The argument that a Form 4473 isn't a "financial record", when it provides documentation of all of the information except the price on all of the dealer's firearm sales (Manufacturer, Model, Serial Number, Type of Firearm, Caliber or Gauge, number of firearms sold, and check boxes for tax exemptions), and the fact the some of the federal firearm excise taxes due don't even depend upon the price of the firearm, isn't a very strong one.
Even if the search warrant didn't single out Forms 4473s from other kinds of business records, this would probably just be harmless error, because the IRS absolutely has the right to ask for and seize Form 4473s in connection with the tax fraud investigation, just like any other business record of a firm suspected of not paying its taxes. These records aren't protected by any evidentiary privilege in a federal tax fraud prosecution.
Also, combined with past sales fliers and catalogues and business records about sales pricing for different products from the dealer in its accounting records, it would be fairly trivial to use this information to recreate a very accurate forensic reconstruction of the gross firearm sales revenues of the dealer and the amount of excise taxes that should have been paid. It would be tedious work, but it would be extremely damning evidence of tax fraud if the estimated sales significantly exceeds the sales reported on the dealer's tax returns.
Even if the some of the documents in a particular banker's box aren't within the scope of the search warrant (for example, perhaps someone put Christmas Cards and as well as accounting ledgers and cancelled checks in the same box), the IRS would not be beyond its rights to grab all of the banker's boxes of documents, review them at their leisure in a government office, and then return the contents of the boxes that turned out to be something other than what the IRS requested a search warrant to seize.
A good faith belief that the boxes seized has some financial records in them would justify taking them away, reviewing their contents, and returning materials that were beyond the scope of what was sought. The IRS criminal division agents don't have to look through the many, many banker's boxes page by page at the dealer's place of business to screen them at that time at that level of detail. Similarly, the IRS agents are not required to assume that boxes actually contain what the label on the outside of the box says that the box contains.
Of course, if the Form 4473s corroborate the tax returns filed by the dealer apart from minor clerical errors or uncertainties about the exact sales prices of sales reported on them due to irregular discounts provided by a dealer who sometimes haggled over the prices of used firearms, the dealer would be vindicated and the informant who triggered the investigation and prosecution (if there is evidence that this informant willfully lied to IRS investigators) might even be prosecuted for making an intentionally false report to a law enforcement officer.
The dealer wouldn't be entitled to reimbursement for criminal defense expenses or harm to the dealer's reputation, but it would still be a huge PR coup and the criminal charges would go away, probably long before a trial was even held.
Montana Attorney General Austin Knudsen absolutely knows all of the facts in the post above. His claimed fear of a crackdown on gun users is something he is doing to willfully mislead the people of Montana for political gain. He may also be throwing stones at federal prosecutions because he was irritated that the IRS and Justice Department didn't keep him in the loop on this tax investigation in his state which he sees as his turf, even though it was purely a matter of federal tax law violations which his office didn't have jurisdiction over. Indeed, the IRS may have kept him in the dark and out of the loop from this investigation, in part, in order to protect taxpayer and gun owner privacy by not sharing confidential IRS investigation information with state law enforcement officers, something that it is not allowed to do.