A good friend recently got his hands on the 10 Eichelburger Squirrel. You can see it beside a .40 S&W for size comparison above.

A necked down 22 Hornet case using a .103 diameter bullet that is solid copper for a velocity of a hair over 4,000 supposedly.

Ten calibre loads such as the 10 Eichelberger Squirrel is based on a shortened Hornet case and will shoot a 7.2-grain bullet at nearly 4000 fps with only 6.5 grains of powder. Similarly the 12 Eichelberger Carbine based on a .30 Carbine case can send a 10-grain bullet at over 4350 fps with 13 or so grains of powder..

You can read more about this little wildcat at the link below if you have become insane enough to dip your feet into the world of wildcat cartridges.

https://www.gunmart.net/ammunition/reloading/bruce-potts-wildcatting