I think I recall the .55 caliber Boys anti-tank rifle being available through Ye Ol Hunter's Lodge prior to 1968. A German variation in a souped up Mauser too. Both of these disappeared from the market with the '68 GCA or a similar act limiting the maximum diameter of the projectile to .5". Yet not one reported used in a crime before or since. Well, concealment would be a problem.
Of course black powder guns are not classified as "destructive devices" and can be purchased in smoothbore up to .69 and rifled .58. Probably more but I'm thinking Brown Bess and Springfield/Enfield.
But sure to cause the anti-crowd a hissy-fit is that the nominal bore diameter of a 12 gauge shotgun is .73. Some states require "slugs only" shooting meaning a projectile almost a quarter inch larger that .5. And some slug barrels are rifled with rifle sights. Even 'scopes.
BATF sez this arrangement is not a destructive device.
Nice to know.