The fact something is, or is not, available for purchase through mail order doesn't make it legal for a person to possess. There's nothing physically stopping a felon or other "prohibited person" from buying ammunition or reloading components "through mail order from Cabelas and other such places" or simply doing so at a local Walmart. The instant such a person is in possession of a single cartridge or ammunition component, he commits a federal felony under GCA-68. Any person can buy a radio scanner by mail order; the last time I checked it's a crime in two states for a felon to possess such a device. When I lived in California, there were stores which would sell a billy club to anyone wanting to buy one. It's a felony in California for a non-LEO to merely possess a billy club.
In numerous states, there are state laws which expressly contradict that statement and people have been imprisoned for years for blindly following such advice. As far as federal law, good luck explaining your notion to Leonard G. Green, Jr. He was prosecuted by the Feds for being a felon in possession of a .36 caliber black powder revolver. The BATF conceded the weapon did not constitute a firearm as defined under GCA-68; that didn't matter to the U.S. Attorney's office or the court. The judge, citing pertinent case law, ruled that black powder firearms are firearms. See
U.S. v. Green, 515 F.Supp. 517 (D.Md. 1981).
What possible use is a black powder firearm when a felon can't legally shoot it?

Possession of any quantity of black powder by a "prohibited person" is a federal felony under 842 U.S.C. § 842(i)(1). Possession of a propellant other than black powder would constitute illegal possession of an ammunition component under GCA-68.
The example you cited obviously involved interstate commerce. In the cases I referred to previously, none of the firearms or ammunition had done any interstate travel before retail sale. The S&W handgun had been manufactured and sold within Massachusetts; it never left the state. The box of Federal rifle ammunition had been manufactured and sold within Minnesota; it never left the state. In every such case I've studied, the defendant was successfully prosecuted by the U.S. Attorney's office.
While ignoring the Second Amendment, judges from U.S. magistrate and district courts all the way to SCOTUS (as do legislators responsible for such laws) artfully misconstrue the Constitution's interstate commerce clause as a pretext for authority for anti-gun (and anti-knife) statutes. In the case of firearms manufactured and sold within one state, judges ruled since the iron ore used in the production of the steel came from another state, that was a sufficient "nexus to interstate commerce" to uphold a GCA-68 prosecution. In the case of rifle ammunition manufactured in Minnesota, the Sierra match bullet used by Federal was produced in another state and that was all the judge needed.
TOM1960, I'm not trying to nitpick or look for an argument with you. I'm trying to prevent people from suffering dire consequences by following (all too common) erroneous legal advice. Contrary to what you wrote, in many states there are procedures in which a person convicted of a state (not federal) felony can get his or her "privilege" to possess firearms restored without receiving a governor's pardon. This has to be done very carefully and, under
Caron, only a full restoration of all firearms privileges will exempt a person from a GCA-68 prosecution.
Since the possession of any quantity of black powder by a "prohibited person" is a federal felony, why would a well-intentioned felon want to own a black powder firearm he couldn't legally use? Some state anti-gun statutes expressly forbid possession of black powder firearms by felons. As the defendant in
Green discovered, all federal prosecutors and judges don't share your interpretation of whether black powder firearms are legal under the U.S. Code. Even in states with laws which "allow" felons to possess antique and/or black powder firearms, I've read numerous cases in which conniving judges found ways to circumvent the language in the statute and convict defendants who peaceably possessed such arms.
I am in no way defending any type of restriction on the possession of any arm by any person anywhere. My intent is solely to prevent people from needlessly facing legal jeopardy by following inaccurate advice. If you study federal case law, you'll find example after example of people whose anti-gun convictions were affirmed by appellate courts despite being previously assured by attorneys, sheriffs, chiefs of police, etc. that it was legal for them to possess the firearm in question. The fact the defendants relied on the advice of attorneys and/or LEOs at the time didn't matter one whit later in court when they were convicted.