The 9×25mm Dillon is a pistol wildcat cartridge that was developed for use in USPSA/IPSC Open guns.
Randy Shelley of Dillon Precision developed this cartridge by necking down 10mm Auto brass to 9mm. The idea was to put as much slow-burning powder in the case as possible in order to drive a 9mm bullet to the velocity needed to qualify for the then-IPSC Major power factor of 175. While the cartridge was powerful and had a higher powder capacity than the .38 Super, it proved to have massive recoil.
There were several notable users of the round in IPSC competitions, such as Jack Barnes and Rob Leatham. Leatham himself developed loads with less blast and shock to mitigate the recoil, only to discover that there was little advantage over a similar load in a .38 Super.
A 115 grain bullet with a muzzle velocity of 1800 feet per second, while impressive, proved to be more than what was needed for competition use. The standards at the time dictated that a 115 grain bullet only needed to go over 1500 feet per second to make Major. What competitors using the cartridge in the late 80s and early 90s were doing was adjusting the powder used, to produce more pressure in the compensator. Competitors could "feed" the comp more gases for higher amounts of force, meaning higher velocity shots. The resulting muzzle blast created by the combination proved taxing over the course of a match, meaning that the recoil re-direction proved hard on the shooters' elbows and shoulders, even causing tendinitis in some cases.
Causing the 9×25mm Dillon to fall out of competition usage was component wear, magazine capacity, and the USPSA lowering the USPSA Major power factor from 175 to 165 (160 for IPSC competitions). The lowered power factor in Major allowed for .38 Super loads to flourish, thanks to their higher magazine capacity and improved effectiveness and safety that the 9×25mm Dillon was no longer competitive. Pistols using 9×25mm Dillon ammunition, when worn out, were often retired or were rebuilt in .38 Super.
As of 2016, 9×25mm Dillon rounds are still commercially available, with DoubleTap offering loads ranging from 80 to 180 grains, and Underwood Ammo offering loads from 90 to 125 grains. In addition, drop-in barrel conversions for the caliber are available for the Glock 20 and Glock 29 pistols, which used 10mm Auto, the 9×25mm Dillon's parent cartridge.