$20 in 1800 is not $500 today.
The most effective way to compare costs across more than 200 years is to convert the price to days of labor.
How many days of semi-skilled labor did something cost then and what is the cost today for that number of days of semi-skilled labor?
A moderately skilled worker in 1800 earned about $.58 per day or about $12/month.
At $15/hour a current, moderately skilled worker earns about $2,500/month, gross.
A month’s semi-skilled work then: $12. A month’s semi-skilled work today: $2,500
Prices for new rifles were about $12 then or the real equivalent of about $2,500 today.
I wrote about what the 2nd amendment meant to Hamilton and the other Federalists who wrote it based on the Federalist Papers and the language of the amendment itself. That’s not a personal interpretation.
Yes, cannons could be privately owned because, without a strong navy, the government depended on privateers to sink enemy ships.
That doesn’t mean that today American citizens should be able to own armed warships.
Your argument that because Madison approved of the government giving the owner of a sailing ship explicit permission to own a cannon to sink pirate ships means that Madison would think that today any untrained, unlicensed, anonymous person should be able to own an unregistered, high capacity, high rate of fire assault weapon is beyond ridiculous.
There may have been a few unique guns that had some sort of multi-shot capacity at various times. They weren’t generally available and the very few that existed were extremely expensive.
The guns that were widely available were single-shot rifles that cost the average skilled worker a month’s labor and those were the guns Madison was thinking about when he wrote the Second Amendment.