First of all, would it be a good idea to use the term "ML" rather than "BP" because the latter can also refer to cartridge firearms pre 1899. Zonie is very fair in that he allows reference where it clarifies the discussion.
Let's go right back to the first page.
624.712 DEFINITIONS
Subd. 3. Antique firearm. "Antique firearm" means any firearm, including any pistol, with a matchlock, flintlock, percussion cap, or similar type of ignition system, manufactured before 1899 and any replica of any firearm described herein if such replica is not designed or redesigned, made or remade, or intended to fire conventional rimfire or conventional centerfire ammunition, or uses conventional rimfire or conventional centerfire ammunition which is not readily available in the ordinary channels of commercial trade.
Subd. 12. Exclusions. Except as otherwise provided in section 609.66, subdivision 1f, this section shall not apply to transfers of antique firearms as curiosities or for their historical significance or value, transfers to or between federally licensed firearms.
This underlined section is the crux of the matter IF the "firearm" is NOT held for that reason. The possession of caps, powder and ball would be held to be evidence of intention to use it.
This is just like the UK, where one can hold an antique firearm as a "curiosity or ornament" and then it is subject to the exemption, (but it is still a "lethal barrelled firearm") the difference being that we don't have the "magic date" for reproductions/replicas. The reference above is a straight copy of the Federal Laws. The "remade ..... " caveat refers to conversion cylinders etc... and at present a strict application of the part referring to ammunition is under threat because now many "Cowboy Action Shooters" are able to buy new ammo in calibres (oops! calibers) which WERE not readily available.
Local jurisdictions can confuse the issue -- even down to city level, where in some places the Sheriff is a dictator. When I used to be in MA I could never undersand why their State Laws required different authority if (e.g., ) a pistol had a larger magazine than 10 rounds. Of COURSE no-one would swap magazines, would they?
All that we can do is to follow the law where we have to and to firmly resist changes. Breaking the law (however unfair or unreasonable) just PROVES to the "antis" what they are trying to justify.