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Since the post-WWII years, if not before, there has been an ongoing argument concerning whether breech thrust (bolt thrust) is reduced by the improved case design. P.O. Ackley has certainly influenced the argument. The definition of an improved case is pretty simple. The case body is blown out to minimum body taper, which is described by Ackley as 0.0075 per inch taper. Shoulder angles between 28 and 45 degrees are normally considered to be improved, although it could be argued that any shoulder sharper than the original parent case is improved. Finally, an improved design allows the firing of a factory cartridge in order to fireform the brass for the new design.

…a method of recording breech trust was necessary in order to go beyond the somewhat subjective experiments that P.O. Ackley wrote about in Handbook for Shooters and Reloaders Vol. I. There Ackley used a Model 94 Winchester because, as he stated, “We often hear that the Winchester Model 1894 action was designed for low pressures and is an action which could be described as ‘weak.’” The purpose of his experiment with the ‘94 was to prove that the improved case design minimized bolt thrust; that the brass will support and contain some pressure; that oily chambers increase bolt thrust; and finally, the notion that actions are designed for specific pressure ranges is a fallacy.