Crimping apparently remains a mystery to many handloaders. One recently asked about "crimp diameter," the theory being that crimp was something that could be measured, just like bullet diameter. It would be a wonderful world if crimping were that easy.

Unfortunately, there really can't be any set crimp diameter for a certain round, because of two variables: thickness of the brass at the mouth of the case, and the deepness of the crimping groove (cannelure) in the bullet. These vary from brand to brand, and even lot to lot, so the same degree of crimp can produce measurably different results, depending on the components used. All of which is why we use fairly vague terms, such a light and heavy, when describing crimp, rather than an exact measurement.

There are two reasons to crimp metallic cases. First, crimping helps hold a bullet in place, important in repeating rifles and handguns that have significant recoil. Note the helps. Crimping alone will not hold a bullet in a .44 Magnum revolver case or in a .458 Winchester Magnum rifle case. The neck of the case must also be sized small enough to help hold the bullet through friction.

Second, crimping can affect accuracy, especially with faster-burning powders. A heavy crimp tends to even out pressures, because the powder must first create enough pressure to "pop" the crimp. More even pressures often result in finer accuracy.

There are several kinds of crimp. Some factory rounds, particularly in Britain, feature a "point" crimp, a few indentations in the case neck around a bullet. This isn't the most consistent type of crimp, however, and would be difficult for the home handloader to reproduce, so handloaders use two other methods: roll crimping and taper crimping.

Roll crimping results in folding the mouth of the case into the bullet's cannelure. Most rifle and revolver seating dies are designed to roll crimp by turning the die into the press until the crimping ledge starts to bend the case mouth. One reloading manual suggests turning the die down until it touches the case mouth, then turning it in another half turn for a "light" crimp, or a full turn for a "heavy" crimp. This is an approximation, however. Some experimentation is generally necessary. It also logically follows that a consistent crimp (necessary for the best accuracy) can only be accomplished by using the same brand and lot of brass, and trimming it to exactly the same length every time. Some rounds require – or function best – with a taper crimp. Instead of folding the mouth of the case into the bullet, the case mouth is pressed around the bullet. Taper crimps are most often used with rimless pistol cartridges. These "headspace" (are stopped from sliding farther into the handgun's chamber) on the front edge of the case, which bumps up against the front edge of the chamber. A roll crimp would result in erratic headspacing, so a taper crimp is used. Here trimming each case every time is even more critical to sure functioning and accuracy.