Yes, I heard the F.B.I. bought a couple of the Anzio rifles.Anzio Ironworks makes a 20mm rifle.
I can remember back prior to 1968 when Interarms and other military surplus gun dealers were advertising British Boys and Finn Lhati 20mm anti-tank rifles by mail order in Guns & Ammo and American Rifleman.
There will never be any of these things for sale to civilians here any more, the potential for some BG getting hold of one and sending high-explosive armor piercing shells at things like armored cars, police cars, fuel depots, power plants, armored limousines, etc. is just too great.