Armor piercing cannons

>>They don't have to be blackpowder
This is correct. Smokeless is OK too, it simply must be a *muzzle-loader*

Also, sabots certainly are a thing for black powder hunters. There are many commercial products on the market. They aren't getting to modern AP sabot ammo speeds (3500fps or more) but it proves that sabots can be used for black powder. They won't go as fast as smokeless but might be good enough.

>Of course they're also gimped by shooting heavy round balls usually of a rather large caliber for what amount of powder they had.
That was a deliberate design choice. There's a pretty hard upper limit on the velocity of a BP gun, so if you want to make a gun shoot farther or hit harder your only choice is to fire a heavier projectile. Round balls were common since that was the standard tech for the time. Shells did exist, of course. Even for shoulder-fired rifles. Some fancy big-game rifles from the end of the 19th century came with molds to cast hollow shells which would then be filled with an explosive mixture.

To be frank, even a tiny 3 pounder iron ball is going straight through an armored car. AP sabot could possibly penetrate an APC.

Careful when loading smokeless into a cannon designed for blackpowder. Smokeless is just way more powerful so there's a good chance of the cannon exploding.

of course you don't want to go around ramming smokeless in an antique cannon, but if its a modern one that can handle the pressure that's a different story.
Keep in mind that you can readily buy surplus cannon barrels that are already rated to handle smokeless pressures...as in a genuine milsurp tank barrel.

is armor piercing absolutely necessary, OP? regardless of penetration, most anything shot out of a cannon will incapacitate jaywalkers. would you be satisfied with a cannon that simply makes dents in people?

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Yes. armored vehicles. Tanks.

>even a tiny 3 pounder iron ball is going straight through an armored car
youtube.com/watch?v=JEKyq7BxTKg
Nah, but there is enough energy there it could be close to a WWII era anti tank rifle

Is the law really as simple as being about muzzle loading, and allows smokeless powder? If so, you could theoretically building a completely modern muzzle loading cannon. It would have the same performance as a modern cannon, but be a pain to load.

Two different definitions at work.
Any actual antique or replica that fires non-available ammo is fine.
As is any firearm that is using an obsolete ignition systme like caps or mathces.
The muzzleloader exemption only counts when using BP or BP substitutes.

You mean for armored vehicles right? Because a 3lb cannon ball regardless of its AP abilities will probably effectively neutralize anyone wearing plate carrier.