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"CRIMP JUMP" or "BULLET CREEP"

“CRIMP JUMP” or “BULLET CREEP”

In 1983, when Freedom Arms brought out their new 454 Casull revolver, I bought one.  As I worked up maximum loads, I noticed that after firing a few rounds, the remaining cartridges in the cylinder had become longer because the tremendous recoil…… As that recoil jerked the revolver back toward me in a split-second, it would act as a kinetic bullet puller, because that heavy bullet wants to stay where it is as the gun snaps/recoils back. The new 454 cartridge was generating more than double the free recoil of the most stout 44 mag. loads of the day and that new Freedom Arms model 83 weighed no more than the typical Ruger Super Black hawk. When you combined that semi light weight revolver, with the super heavy recoil, you would get “crimp jump” or “bullet creep”.

By the mid 80’s, I was loading for and shooting the 500 Linebaugh revolvers that featured 440gr.+ bullets. These loads generated more recoil than the heavy 454 loads in a revolver of like weight and when you factor in that very heavy bullet that wants to stay put under recoil, “crimp jump” became a very real problem for revolver reliability.

If the bullet creeps forward out of the casing too far, (protruding past the cylinder face) it will hit the back of your barrel, when you cock (rotate your cylinder) the revolver, thus tying up the gun and rendering it inoperable.

Today we have all types of light weight revolvers chambered in 357 Mag. or 44 Mag.. Some of these alloy 357 revolvers weigh as little as 12 oz. When you combine such light weight with the free recoil generated by even modest 357 Mag. loads, you run the risk of getting the bullet to “crimp jump”.

CURES FOR CRIMP JUMP

There are several cures for “crimp jump”. A serious roll type crimping die, set to administer a serious crimp is the biggest part of the formula in eliminating crimp jump. However, bullet makers also need to change the decades old style of crimp groove too and then if you adjust your crimp die to fit such an improved crimp groove, you can all but eliminate dreaded crimp jump. At Buffalo Bore, we have improved all of our cast bullet designs to have a very aggressive crimp groove. Even in the lightest revolvers, our customers no longer experience crimp jump with our modern hard cast ammo, because our crimp dies are designed to match our aggressive crimp grooves. Most jacketed bullet makers, still have much to improve in the design of their crimp grooves. The Barnes all copper revolver bullets, have a very improved/aggressive machined crimp groove, which when coupled with a proper crimp die, will eliminate crimp jump.

HOW USER ERROR CAN CAUSE CRIMP JUMP

User error also contributes to crimp jump in some cases. How so, you may ask? As an example, let’s say you are firing a five shot alloy J frame revolver and you fire only three rounds and then reload your cylinder. You should remember to then rotate that reloaded cylinder so that the two remaining rounds that were unfired, are now in line to be fired first upon the next firing. If you fail to rotate your ammo thusly, you might keep exposing the same unfired rounds to more and more recoil impulses, thus insuring you’ll eventually get those rounds to jump the crimp.

BULLET SET BACK

Bullet “Set Back” is the opposite of “Crimp Jump” and the two should not be confused. “Set back” happens when the bullet is driven deeper into the casing from being in a magazine of any sort, under recoil. This can happen in bolt action rifles as well as lever action rifles. When the rifle is fired and undergoes recoil, the cartridges in the magazine get slammed into each other, end for end (in a tube) or get slammed on the bullet nose, into a magazine box wall. Whether in a tube or a magazine box, that slamming on the bullet nose, can drive the bullet deeper into the casing. These dynamics are basically the opposite of “crimp jump” as the over-all-length of the cartridge is now getting shorter, as opposed to longer. This situation can cause cycling/feeding, high pressures and/or accuracy issues and can be remedied by using lessor recoiling cartridges in heavier rifles. Other remedies would include using a compressed powder charge, so the bullet cannot be driven more deeply into the casing or using enough crimp can also alleviate the problem to some degree. Sizing your case necks down enough to give tension on the bullet shank, can also aid in eliminating the bullet being driven more deeply into the case.

Bullet set back is also common in semi-auto pistols. It is caused as the cartridge is chambered. Chambering in a semi-auto is violent to the cartridge as the bullet nose is slammed into the feed ramp. See my essay on “Bullet Set Back In Semi-Auto Pistol Cartridges”—“causes and dangers”.

Hopefully this short essay can be of some help to our customers in understanding the dynamics of dreaded “crimp jump”.

God bless and good shooting,

Tim