What’s your interpretation of easy? These types of firearms aren’t cheap. For example, a Barrett .50 BMG M82A1 is like $12,000. No website is going to ship it directly to you either. It has to go through an FFL which will perform a background check.
(f) For the purposes of the National Firearms Act, the term “Destructive Device” means:
(2) Any type of weapon by whatever name known which will, or which may readily be converted to expel a projectile, by the action of an explosive or other propellant, the barrel or barrels of which have a bore greater than one-half inch in diameter.
Which is how >0.50" black powder rifles, air guns and flare guns still exist.
[25.4 mm == 1 inch]
there was an issue with the desert eagle as a 50 cal weapon, at time considered a DD, so there was a revision for civilian carry that was, 49.5 cal with a tapered chamber. if bored out to a true 50 cal, it would constitute production of a DD
i think i was looking at the 20mm cannon as well
https://news.ycombinator.com/item?id=32957960
but posted over here.