Federal law and most state codes prohibit certain people from carrying firearms, or even possessing them. These are "Prohibited Persons": convicted felons, individuals under a court order for reasons related to domestic violence, and some others, including people under 18 years of age.
It's possible to become a convicted felon, and thus a "Prohibited Person," without having committed any sort of violent act. Insider trading or jiggering the account books at a business are non-violent offenses that can saddle someone with a felony record despite their being no threat to anyone. It's not my goal here to argue that felons or anyone who qualifies as a "Prohibited Person" should carry a gun but the fact is that such individuals can obtain and own some sorts of guns legally; some of them will no doubt do so and perhaps will carry them (though if they have any sense it will be concealed).
Under Federal law, and in most state codes that mimic its language, guns using loose powder and ball (i.e., "muzzle-loading" guns) are not "firearms" in a legal sense. They're described as "antique firearms" completely exempt from all provisions of the never-sufficiently-to-be-cursed and clearly unconstitutional Gun Control Act of 1968. In US Code 18 Paragraph 921(A)(16) and in 27 CFR 478.11, the definition is provided:
Antique firearm: (a) Any firearm (including any firearm with a matchlock, flintlock, percussion cap, or similar type of ignition system) manufactured in or before 1898; and (b) any replica of any firearm described in paragraph (a) of this definition if such replica (1) is not designed or redesigned for using rimfire or conventional centerfire fixed ammunition, or (2) uses rimfire or conventional centerfire fixed ammunition which is no longer manufactured in the United States and which is not readily available in the ordinary channels of commercial trade.
Furthermore, the BATF's "Question and Answer" page says this about whether a background check is needed for such guns:
Is a NICS background check required for the transfer of antique firearms?
No. Because weapons that meet the definition of an “antique firearm” are not firearms subject to the Gun Control Act, licensees need not conduct a background check when transferring an antique firearm. [18 U.S.C. 921(a)(3), 921(a)(16), 922(t); 27 CFR 478.11 and 478.102]
So in many states anyone, even a "Prohibited Person," can walk into a gun shop and buy, let us say, a percussion-fired revolver, or order one from a catalog to be delivered to his door. It can be argued whether this is a good or a bad thing, but it is the law.
Now let's say that a non-violent convicted felon (e.g., someone convicted of "insider trading" or of "cooking the books" at a business, someone who is not any sort of danger to the community, feels a need for protection. Could he or she then buy an "antique firearm" as defined in the law, to use for personal protection? The answer of course is "yes."