Introduced in 1963, the .221 Fire Ball and its Experimental Pistol Number 100 were the brainchild of Remington's Wayne Leek. An abbreviated single shot version of Leek's Remington Model 600 Carbine, the pistol was introduced to the shooting world as the XP-100. For shooting varmints at long range with th .221 Fire Ball, 50 grain bullets with extremely thin jackets are the very best choices. Good examples are the Nosler Expander, Hornady Super Explosive, Sierra Blitz, and Speer TNT. H4227 is an excellent powder for this mild mannered little cartridge. 

This text is based on information from “Cartridges of the World”, Hodgdon reloading manual, the cartridge designer and/or own resources.