The .20 Caliber Pellet
Sheridan introduced the .20 caliber pellet to the world in 1947. Then they also referred to it as a 5mm. In 1947, Ed Wackerhagen designed a multi-pump pneumatic air rifle that he called the Sheridan. He found commercial airgun ammunition of the time too inaccurate to work well in his rifle, so he created a proprietary caliber, the .20. No other airguns were made in that caliber until Robert Beeman requested Feinwerkbau to make up five special model 124 rifles for his company. That project never went anywhere, but within a few more years Weihrauch, the German maker of all the Beeman R-series rifles, began making 5mm guns and the market blossomed from there.
America has been the leader in .20 caliber/5mm airguns, but Europe is now producing more of them all the time. The pellet makers are also making more designs of pellets in this caliber. There are still fewer choices in .20 caliber than in .177 and .22, but the gap is narrowing.
Many shooters consider .20 caliber to be a good compromise between .177 and .22. way in his Actually, .20 is a little closer to .22 than it is to .177 in terms of the cost of the pellets and pellet weight.