Just looking back at one of the posts & saw a mention of Flobert. I've seen references say that the rimfire bb cap he invented was from around 1840, in .22, so this could have established a market that believed .22 was the way to go for indoor training/fun shooting. Hence when someone wanted to get into that market with an air rifle the .22 would have been the way to go as it was already an accepted calibre for thst purpose. Dont know why Flobert adopted the .22 in tbe first place though.....