Every modern plastic or paper shotshell has a crimp. And all of them have crimps for the same principal purpose: to keep the shot and ejecta inside the hull until combustion commences and is completed.
Now, if that were the only reason for a crimp, we could stop right there. But actually the crimp provides several other important functions supporting the successful chambering and ballistics of a shotshell load. And there are several different types of crimps to consider.
The most common crimp today is officially called a fold crimp, whereby the top of the hull is folded into either six or eight segments to form a complete closure. In appearance the closure resembles the precut lines of pie slices, from which is derived the alternate name “pie crimp.” The shotshell manufacturing and reloading worlds have pretty much gone to fold crimps on the vast majority of their loads for two reasons. The first is speed. A fold crimp can be simply and quickly applied with a crimp-starter die followed by a crimp-finishing die with no additional components needed on every high-speed-production loader and shotshell-reloading press. The second reason is that the fold crimp provides the best defense against moisture entering the hull at the top. And this is a very important feature, because water entering a shotshell frequently leads to bloopers that can leave wads lodged in barrels. At the very least bloopers will result in a lost attempt at that mallard or pheasant you just pulled down on. But much more seriously, if the lodged wad is not detected and removed before a following shot, a blooper-caused obstruction can result in serious damage to the barrel and/or personal injury. In fact, bloopers from water entry are the number-one cause of barrel-damage claims against shotshell manufacturers.
Now notice I didn’t say fold crimps provide a means of sealing the crimp end. That’s because no crimp is waterproof, as it would be if truly sealed. And by the way, if you want to make a million, put your time into inventing a quick and cheap way for fold crimps to be rendered waterproof. As of today, no one has ever succeeded in doing so. Yes, I know that some manufacturers and reloaders apply little gobs of wax or various liquid “sealants” or fingernail polish in an effort to seal the crimp. But none of these materials truly seal, they often are brittle and they don’t adhere well to the slick surfaces of plastic crimp folds, usually leaving one or more “holes” that permit water entry. Winchester has abandoned liquid-based sealants altogether and has elected to try to up the weatherproofing of its crimps by running a little spinning, arrow-type, broadhead-looking knife blade into the center point of many of its fold-crimp loads. This leaves a little circular melted-looking crater in the crimp middle. But frequently these little craters also have holes.
Which brings us to roll crimps. Roll crimps are the old-fashioned way of providing a closure to the end of a shotshell. Other than providing more room in a shotshell, there is nothing advantageous about a roll crimp. First, roll crimps are not sealed and, in fact, would be even more difficult to seal than a fold crimp. Second, they provide no improvements on ballistics. Third, they require an extra component—the infamous overshot-card wad—which, if too stiff, does not fragment properly, exits the muzzle in front of the shot and contributes to a spreader-load effect. Fourth, they take more time to form than a fold crimp and thus do not lend themselves to being assembled on high-speed factory-type loaders.
With all the above in mind, it becomes clear that the lowly crimp is really something quite above lowly. And besides impacting all of the above, the crimp also has another important function. A properly formed fold or roll crimp has a slight bevel around the entire crimp rim. This greatly aids successful chambering of shotshell. It is especially important with autoloaders and pumps, where multiple rounds are mechanically transported from the magazine to the chamber instead of being manually loaded as with double guns. Improperly formed crimps that are inadequately beveled at the crimp mouth are the number-one cause of poor chambering of reloads.
So all reloaders would be wise to quickly learn how to adjust the typical reloading press’s crimp-starting and -finishing dies and be prepared to do so whenever changing loads. Probably the number-one mistake reloaders are guilty of is leaving the crimping stations on their reloading presses permanently set to what they were when they came from the press manufacturers instead of learning how to adjust the crimp, which almost always has to change from load to load, if one wants the best crimp. And that’s why factory crimps look so good.
Instead, all too many reloaders have not learned the lesson to never put card or plastic wads on top of the shot to adjust crimps. To repeat: Any card or plastic component loaded on top of shot that is not completely frangible always imparts a spreader-load effect to the pattern. If that’s what you want, go for it. But if you want the best patterns, especially at long range, never put anything on top of the shot. Always adjust crimps by adjusting the reloading tool and/or, if necessary, add card wads under the shot.
Last, fold crimps should have all of their fold points touching, so that there is no little hole in the crimp center. Such holes can allow small-size pellets to exit the load and water to enter. Also, it is a myth that crimp folds should be flat. I don’t know where these kinds of hearsay rumors start in the shotshell world, but they are all too common. Fold crimps do not have to be flat. If you look at Remington factory loads, for example, all of the fold crimps on their loads tend to be slightly dished downward. Most other manufacturers opt for basically flat crimps, and some even turn out loads with slightly dished-upward crimps. But the key word here is “slightly,” as crimps that are too raised in the middle have a tendency to open under the vibration of being carried in pockets of hunting coats, vests and so on, which then allows shot to escape.
So that’s it. The seemingly little old crimp is actually a big deal.