There is a belief among some people that applying a really tight crimp, so that the case mouth “grips the bullet tightly,” is how we avoid bullet setback as the bullet tip hits the feed ramp while feeding into the chamber. This is false. What actually prevents bullet setback is (1) friction between the bullet and the interior walls of the cartridge casing, (2) what has been termed “the shelf.” (For years I called it “the cup.”) What is this, and why is it important?
When we fire a cartridge, powder inside the casing burns, producing a large quantity of expanding gas. When contained inside the casing, which is surrounded and reinforced by the steel chamber, this gives us five digits worth, pounds-per-square-inch-wise, of pressure which serves to force the bullet out of the casing and down the barrel, thus, to exit the muzzle at hundreds or even thousands of feet per second. All else being equal, the smaller the interior volume of the casing, the higher the pressure, and the faster the pressure spike (how fast the pressure attains maximum poundage). Exceed how much pressure the casings can take, in a self-loading action, we can wind up with a case blowout in the unsupported area over the feed ramp. And one way to lessen case capacity dramatically is to have the bullet be forced back into the casing during feeding.
Now, how to avoid that as a handloader? Why, I’m glad you asked!
As a cartridge casing is resized, it winds up a bit undersized compared to the bullet diameter. Therefore, it’s necessary to slightly flare the case mouth so it’s larger than the base of the bullet, in order for the bullet to enter the case mouth and be driven downward into the casing at the bullet seating station.
As the bullet is seated, it expands the casing, but only down as far as the bullet seats. What we wind up with is the material of the cartridge casing, for all intents and purposes, folding inward under the base of the bullet, providing a “shelf” for it to sit on. In combination with the aforementioned friction between the entire circumference of the bullet and the surrounding sides of the interior of the casing, the shelf prevents the bullet from being driven back into the casing during feeding.
There are two primary ways we can compromise the shelf (NOTE: We do NOT want to do that – these are mentioned as things to avoid):
(1) Overflare the case mouth, i.e. open up the case mouth too much, in pursuit of making it easier to center and seat a bullet at the bullet seating station. Do that, and when our crimp die “squishes” the top of the casing at the crimp station, there will be an area underneath the crimp that’s still bowed outward, thus does not give us the amount of friction we need between the sides of the bullet and interior of the casing to resist bullet setback.
(2) Overcrimp the case mouth. Especially in combination with overflaring the case mouth, this also causes the area under the crimp to bow outward.
At the extreme, overflaring combined with overcrimping can cause the area of the casing that would normally curve inward to create the shelf to instead bow outward with no shelf. Don’t overdo either case flare or crimp. Only enough to get the job done and no more is the rule.
