General crimping question


#1

This ammo has a 4 stab crimping arrangement. Others have 3 crimps. How do manufacturers come up with which crimp to use? Are there 5 or 6 crimp rounds? Are there 2? Is there relationship between amount of crimps and air tightness?
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#2

Vlad, I have no answers to all of your questions but one of the reasons I know of for about 8 crimps was on a 9x18 Makarov case which had an oversized primer pocket. Instead of scrapping these the just got plenty of crimps around the primer. I guess the “oversize” on these cases was just above the allowances. Otherwise I can not imageine how that would have worked.


#3

I have a Kynoch .450 Black powder Blank with six crimps. These blanks were made for use in pistols that had been converted into blank only by welding a steel rod into the barrel. The heavy crimps were required to prevent the primer blowing out due to the excessive pressure caused by the blocked barrel.


#4

[quote=“sksvlad”]This ammo has a 4 stab crimping arrangement. Others have 3 crimps. How do manufacturers come up with which crimp to use? Are there 5 or 6 crimp rounds? Are there 2? Is there relationship between amount of crimps and air tightness?
[/quote]

sksvlad,

You can find just about any number of stab crimps, Japanese 6.5mm Arisaka had zero, two and three stabs. Somewhere I have WW2 WRA .303’s with three and four stabs and some which went through the crimp process several times, one has twelve times! The principal purpose of crimping it to prevent blowback of the primer. The gastightness (is that a word?) depends of the size, shape and surface finish of both primer and pocket.

gravelbelly


#5

And there are lots of primer crimp styles. Probably the most common is a round crimp around the entire primer. In some cases it is hard to detect the “crimp”.

Gravelbelly said it! The purpose is to prevent primer blowback. Blowback is caused by the chamber pressure, the length of time the pressure is applied, the holes that allow the pressure in and out off the primer pocket, the fit of the primer and the strength of both the primer and case material. It is also a function of how safe the engineers think they need to be.

Cheers,

Lew


#6

Falcon - I don’t know the case with the particular rounds you are talking about, but the truth is that in revolvers, primer set-back is more often a case of too little pressure. When a revolver is fired, the forces at work tend to set the whole cartridge back hard against the flash-shield of the revolver, which keeps the primers from backing out. No matter how fine a revolver is made, there is always a little headspace gap behind the unfired cartridges. If it is too tight there, it can easily cause malfunctions of the revolver by jamming the cylinder. This can be caused with a slightly high primer (unfired, I am talking about), a piece of lead flash off of a bullet previously fired, residue build up, etc.

If the pressure is low, as in a blank (the pressure escapes out the sides of the cylinder gap at the front of cylinder, behind the barrel’s forcing cone, and doesn’t really need the bore open in the case of blanks fired in revolver-type handguns), it can be enough not to set back the case itself, but to blow out the primer slight, and jam the weapon. The first time I made dummy .38 Special rounds for anyone, and thought I knew everything about what I was doing, I made the dummies by breaking down live rounds. After extracting the bullets and powder, since the person they were being made for wanted fired primers in the cups, I simply put the primed empty cases in my Colt Python and fired the first one. It lock the gun up solid. Fortunately, I was able to diagnose the problem, even though I had had no idea of such a thing before, and found it eash to drive the primer back in with a wood dowel down the bore, into the case, and tapping it with a rubber mallet, thereby reseating the primer.

Short story - primer blowback in revolvers is, my opinion, much more likely to be from low pressure than high pressure.

John Moss


#7

A good example of crimping to avoid backed-out primers in revolver blanks mentioned by John above is the U.S. .45 M9 revolver blank. My specimen has only three triangular crimps around the primer, but they are so wide they crimp over half of the entire circumference of the primer. Jack


#8

John, I am talking about blanks for revolvers where the barrel is completely blocked and there is only the cylider gap to relieve te presure. However, the crimps could also serve the purpose of preventing setback when the blank was used in an ordinary revolver.


#9

Falcon - that’s also what I was talking about, both blank revolvers with solid barrels (or real revolvers made into blank revolvers and with the bores blocked). And yes, the crimps could certainly help the situation of primer back-out caused by LOW pressure. Now, I am only talking about revolvers - I am not talking about blank guns made in the form of an auto pistol, or blanks fired in an auto pistol. In a revolver, the problem is usually low pressure, not high pressure, causing primer set-back, for the reasons I previously described. Matters not at all if it is a real revolver or a blank gun. They both bleed gas at the cylinder gap, and of course, the gun with a block barrel will bleed most of the gas there. There is always residual pressure in the chamber, of course, but often not enough to set the case back firmly against the revolver’s flash shield.

John Moss


#10

In most instances, where you find 6 primer crimps on a 7.9x57mm case it is a result of the original primer being removed and the new primer being staked. This is not always the case however. The firm avu (Silva Metallwerke GmbH, Werk Genthin ) on many case lots of 1945 seemed to have run cases through the staking process twice. If there was a reason for this or if it was simply accidental, I have no idea.

On the other hand, you can find DWM (P131) cases, for an example, that have 3 primer stakes and no primer stakes in the same case lot and loading.


#11

Could it have been to avoid having to scrap slightly undersize primer cups or cases with oversize pockets, especially as it was getting desperate for Germany in 1945?