20mm was a pretty popular caliber for AA guns, and you can often find them in the ground around the cities and military bases in Europe that have been heavily bombarded in WWII. In Europe it's a lot of fun to play with metal detectors since there's a lot of stuff in the ground around old cities, from ancient coins to more modern things like WWI and WWII bullets, broken bayonet blades, helmets, and all kinds of old metal junk.
edit: hmm, maybe that doesn't make sense since he also says 12.7mm, maybe he lives where there was once a war like m0zg says