In my previous article, I indicated the generally accepted preference for roll crimps in revolver ammunition and taper crimps in semi-auto ammunition, and briefly mentioned the potential problems that can be caused by incorrectly crimped ammunition in both types of handguns.
But why is there a difference? n a word: “headspace.” I don’t want to get into a detailed discussion on headspace as it is a somewhat complex topic and depends a lot on the specific type of case you’re talking about, i.e., it varies for rimmed cases, rimless cases, straight walled cases, bottle neck cases and all the possible variations. There are a number of good discussions of headspace on the Internet. If you are really interested in delving deeper into this topic, just Google “ammunition headspace.”
For our purposes, let’s use a slight oversimplification and just say headspace defines where a case is positioned ready for firing.
The semi-auto/revolver issue relates to the use of rimless cases for most semi-autos and rimmed cases for revolvers. As we know, this is a general statement and not always true as there are semi-autos that use rimmed cases, e.g., Desert Eagle, and revolvers that use rimless cases, e.g., S&W 929. So to be on the safe side, let’s refer to handguns that use rimless cases and revolvers that use rimmed cases.
When a round of ammunition is fed into the breach of a firearm, it is precisely located and held rather firmly in place in preparation for firing. The method by which this location is achieved varies for rimless and rimmed ammunition.
For firearms which use a rimmed case, the front face of the rim seats firmly against the cylinder or breach face, stopping the case from moving forward. In this situation, the nature of the crimp around the case mouth is of little importance, at least to case positioning.
However, for a rimless case, the position of the case is determined by the edge of the case mouth which is located against a ridge inside the breach. If the round has a roll crimp, there may not be enough edge on the case mouth to firmly locate the round in place, hence the preferred use of a taper crimp for such cases, which keeps a relatively sharp edge in place to mate with the inside of the breach.
Other Crimping Related Issues for Semi-Autos and Revolvers
Previously I highlighted how the recoil force of revolver may be sufficient to move a projectile further out of the case, increasing the Overall Ammunition Length (OAL). This increase in OAL may be sufficient to stop the cylinder of the revolver rotating. As most revolver ammunition utilizes rimmed cases, and the use of a taper crimp is not required, a fairly aggressive roll crimp is preferred. In fact, projectiles specific to revolver calibers often have a special groove on them to accommodate the roll crimp—more on that in the next article.
For semi-autos, the possible movement of a projectile is somewhat the opposite. The more likely scenario is a projectile being forced further into the case during the load cycle of the semi-auto. This may be due to the force exerted on the case as it is moved forward from the magazine, or the projectile catching on the front face of the magazine or breach face as the round is being forced into the breach. A crimp in this case has mainly to deal with a force in the rearwards direction, i.e., shortening the OAL. A taper crimp is thus the preferred crimp in this case.
In the next and final article on crimping, I want to conclude by discussing the various types of projectiles for semi-autos and revolvers, and how the consideration of crimping varies for these projectiles.