20mm Isn't that huge. The Lahti wasn't much bigger than a Barret .50 cal "sniper" rifle (the Barret is 5 feet long, That Lahti was a bit over 6).

Alas, I'm sure that if they had access under the same terms as regfular rifles, some nut case would set up someplace and start taking out cars & trucks on the freeway. *sigh*