History
The .25 cal. pellet is as old as the .177 and .22. Perhaps, the .22 is a little older than the others, but you have to get into esoteric shapes, such as cat slugs (solid lead bullets with felt bottoms), before that even takes effect. So, for all intents and purposes, the .25 is just as old. But, it was only as popular as the other two in the early 1900s. After about 1914, .22 and .177 calibers took off, leaving the .25 cal. in the dust…until now.
https://www.pyramydair.com/blog/2010...pellet-part-1/
plus this
LEN AND ARTHUR JEFFRIES CONTINUED TO PRODUCE AIR RIFLE PELLETS IN CALIBRES .177, .22 AND .25 FROM THEIR PREMISES IN WHITTALL STREET AND STEELHOUSE LANE, LOCATED IN THE HEART OF BIRMINGHAM UNTIL 1960, WHERE DUE TO THE REDEVELOPMENT OF THE CITY WERE FORCED TO RELOCATE TO A NEW FACTORY IN SUMMER LANE.
http://marksmanpellets.com/history/
another quote here
“In November 1900 the company obtained a number of air gun pellet making machines and the 1901 catalogue indicates these being made in sizes 1, 2 and 3, later to be defined as .177”, .22" and .25". The pellets made by the company were to be named the ‘Match’ weighing 10 grains and the ‘Witton’ weighing 8 grains. These were subsequently added to with the ‘Lion’, ‘Swift’ and ‘Mitre’ pellets."
https://forum.cartridgecollectors.or...istory/11867/4
slugs V pellets
A) At the beginning of the last century most, if not all, ammunition for air-powered firearms was in the form of cup-shaped lead slugs. These were made by a number of manufacturers in various sizes – No. 1 bore (.177in), No. 2 bore (.22in) and No. 3 bore (.25in). Slugs were marketed for use in smoothbore airguns, and pellets were introduced for rifled airguns. They were not fully interchangeable, but pellets dominated the market despite being twice as expensive as slugs.
The cheaper slugs were the most popular pellet until serious target shooting with air rifles
became popular in the early 1900s. When air rifles became more sophisticated, better precision-made pellets took over the market. The cheaper form of the waisted pellet has replaced the old slug and they seem to work fairly well in smoothbore barrels.
http://www.shootinguk.co.uk/answers/...rsus-slugs-525
So you see , .25 is not a newish idea
I would think that there would be some one in the section that can give a better idea