You can own a mortar tube in the US. It's classified as a "destructive device" so you might have to fill out some extra forms, and it's probably hard to find someone who will sell it to you.

Do you know anybody in the 1700s with magfed rifles?

Not that I'm aware, but they did have privately owned cannons firing grapeshot; veritable WMDs. Grenades were also a well established concept back then, though I don't know what the history of private grenade ownership in America is.