Anything 20mm or larger is considered a cannon - an entirely different class of weapons from small arms.
cannon
Smaller than 20mm.
ammunition box
A 20mm floating opal is typically about the size of a nickel or a small coin. It is considered a medium-sized opal that can be easily incorporated into jewelry designs or used as a focal piece in accessories.
About 100 of the early P-51A ( called NA 91 by North American ) were fitted with four 20mm cannon, many of these were sold to the RAF. In general it seems the 20mm guns were not as reliable as the 50 cal, and also the aircraft could only carry a limited supply of 20mm shells which were each about 3 times the weight of 50 cal.
It can be considered a significant amount of rain depending on the context. In general, 20mm of rain in one day is considered a moderate amount for most regions, but it may be considered a lot in areas that typically receive lower amounts of rainfall.
It depends on what you classify as 'cannons' . Many aircraft were fitted with 20mm guns in exactly the same way as machine guns were. Some aircraft such as the Stuka, some other German aircraft and a few British aircraft were fitted with larger guns of 37- 40-mm calibre and these were often carried in pods under the wings and had very limited ammunition due to it's weight.
In guns including firearms, caliber or calibre is the approximate diameter of the barrel and by extension the projectile used in it, measured in inches or millimetres. The 20mm caliber ammo is the biggest one in production.
California has strict gun laws and I doubt that you can legally own a 20MM rifle in California. Usually the only guns available in California are small handguns.
It was considered a scaled down - or miniature - version of the Vulcan 20mm cannon.
mark a pt 20mm above xy planemark 20mm below
The Paris gun of WWI. It weighed 256 tons. it was 28m long and fired a 240mm round. It was made by Krupp of Germany. The German army used it on Paris France.