The 20mm caliber is a size of cannon ammunition, most of which are shells carrying some kind of payload and an explosive fuse. Being a military cartridge, it is typically only available from Old World military bases and cannot be crafted.
This is a custom ammunition type created especially for the NIF-Bashed Weapons Mega-Pack.
There are four varieties of 20mm ammunition available:
The following Mega-Pack weapons can fire this ammunition: