The M18 recoilless rifle was an anti-tank weapon used from WWII through Korea with some limited use in Vietnam.
In the video below, ORDNANCEdotCOM fires .50 BMGs through their M18, which has been registered with the ATF as a destructive device per NFA regulations.
Typically firing a 5.3 lb 57mm fixed round, this M18 features a barrel insert allowing it fire the smaller .50 cal BMG.
The insert appears to have been built from a 57mm cartridge case, which can be see in the first few seconds of the video below.