If you had mad design and machining skillz would it be legal to construct a large bore working cannon or Gatling gun style artillery piece for your own amusement, or do laws limits even weapons you build yourself?
You can buy full-size muzzle-loading cannon barrels through the mail. By ‘full size’ they mean exterior dimensions. I know that a replica Civil War field cannon barrel has a bore of about 3-1/3".
How to Build a 12 pound Mountain Howitzer Cannon
Barrel Price List at Steen Cannons
I’m not seeing anything about licenses, or going to jail.
With a muzzle-loader I think you’re safe.
I am posting this without a cite, but I think there is a .55 or .60 caliber limit if you want to build something that fires fixed ammunition. (i.e., case, projectile, powder and primer all together.)
What about a Naval gun, where a projectile is loaded from the breech and the powder charge is inserted behind it? I’ve got no answer.
What?! No trailer hitches? What kind of…
They did it on MythBusters, and that show always says something about how you should try this stuff at home. So go for it.
Second Amendment, no?
Is it illegal to build a large cannon?
Maybe, maybe not?
Is it illegal to build a large working cannon?
How do you legally find out it works? 
As long as you are allowed to posses it, and are at a location where you are allowed to fire regular firearms, you should be allowed to fire it.
Muzzleloading cannons are unrestricted, as is the rule for muzzleloading weapons generally.
Breechloaders over .50 caliber are classed as destructive devices.
Thanks for looking that up. I didn’t know .50 caliber was the limit, and thought it was slightly larger.
A neighbor of mine once had a small brass cannon with which he would fire salutes on Independence Day. All the neighborhood kids loved it. Don’t think he ever got ticketed for either possession of a banned weapon or disturbing the peace.
How come the guy with the biggest/most weapons is always the most patriotic?
Gatling guns are also not restricted as they aren’t technically a full auto firing weapon.
Full auto being defined by the BATF as multiple shots fired with a single trigger pull.
Gatling conversion for Ruger 22 rifles. A crank and cam mechanism that operates the triggers.
I saw one online once with the conventional circular design that people associate with Gatling guns. Don’t have time to find it again now.
Because patriotism is defined as how much into war and military stuff you are.
Note that that’s mechanical gatling guns (driven by a hand crank or such) not electric ones, which would be considered full auto.
A friend of mine used to have one of those 10/22 kits. It was fun although it was hard to get both guns sighted in to the same spot.
I hate to keep bringing this up whenever a question such as this is posted, but it all depends on what country you live in and, if you live in the United States, probably what state you live.
I don’t have access to most of these from here, but the OP may find these google results interesting: