There are a number of "Improved" versions of the 257 Roberts, developed mostly in the late 1940's and early 1950's. The 257 Ackley Improved is one of the best, and certainly the most popular of the crop. This cartridge has rather a straight, blown out case walls with very little taper and a 40 degree shoulder angle. As with the other Ackley Improved cartridges, cases are made by firing factory ammunition in the Improved chamber. The 257 Improved has about� the ideal case capacity for the 25 caliber and is quite efficient in the velocity it produces with a given charge of powder. The 257 Improved will develop from 100 to 300 fps more velocity than the standard 257 Roberts, depending on bullet weight. In fact, velocities are only slightly below those developed by the larger 25-06 Remington with the same weight bullets. The 257 Improved has proven to be an excellent cartridge for long range varmint shooting and also for big game such as deer, antelope, black bear, big horn sheep, etc. It is one of the best of the Improved line of cartridges in terms of useful velocity and energy gain. 

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