The most commonly used head design today is that of the rimless case. Despite the rimless designation, the case does indeed have a rim to facilitate extraction. In this design, however, the rim does not extend beyond the case body.
Rather, the rim diameter is approximately equal to the diameter of the case body itself just ahead of the extractor groove. Designed for flawless feeding through the various weapon types that were emerging in the late 1890s, the rimless case has become the most popular and widely used head type in the world.
With no protruding rims or belts to complicate feeding, the rimless case has proven itself eminently well suited to later military developments, such as clip loading, magazine and belt-fed weapons. The first successful rimless design adopted by the U.S. was the .30-03 Springfield (redesigned as the .30-06 some three years later), although the U.S. military had been looking at rimless designs for at least a decade prior.
Rimless service cartridges have been the standard ever since, progressing from the .30-06 to the 7.62mm NATO/.308 Winchester to the current 5.56mm NATO/.223 Remington.
Rimless cartridges may be found in either straight-wall or bottle-necked configurations. Headspace in the bottle-neck designs is based upon the datum line, or midpoint of the shoulder.
To the handloader, this means that sizing die adjustment is critical to assure proper and safe functioning of reloaded rimless cartridges. If the shoulder is pushed back by a die that is adjusted down too far, excessive headspace will result.
This causes poor ignition, accuracy problems, short case life, and poses a serious safety hazard. Despite these potential problems, the rimless design is still one of the best combinations for reliable headspacing, trouble-free feeding, and adaptability to a wide variety of action types.
In straight-wall cases, the rimless design has become the standard for semi-automatic pistols, from the time-honored .45 ACP, right up to the most modern designs such as the .40 Smith & Wesson. The straight-wall (and/or tapered) rimless design has also been used for some rifle cartridges, such as the .30 Carbine. In either rifle or pistol, positioning (headspacing) of a straight-wall rimless case is controlled by the case mouth, stopping on a corresponding ledge on the inside of the chamber.
As such, case length is a determining factor in controlling headspace. Positive headspacing can also be influenced by the type and degree of crimp. For example, a .45 ACP case of proper length may headspace reliably if given a slight taper crimp, yet demonstrate excessive headspace if a heavy roll crimp were applied. As a rule, cases which headspace on the case mouth should only be taper crimped, and only slightly at that.
At present, it appears that the rimless case, in either straight-wall or bottle-necked configurations, will continue to be the most common design for the foreseeable future.