Author Topic: Taper Crimp: Everything you need to know  (Read 21169 times)

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Offline Wobbly

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Taper Crimp: Everything you need to know
« on: April 08, 2016, 06:49:17 AM »
Taper Crimp
Everything you ever wanted to know, but never seemed to be documented anywhere else...

Taper crimp continues to be an area of great concern for novice reloaders and a subject that's not well documented for some strange reason. This situation has allowed a LOT of confusing ideas, internet rumors, and outright misinformation to remain in circulation. This article is intended to be a compilation of the most concise factual information from a single-source on the subject.

Every writer comes at their subject from a particular vantage point. I have approached this topic from the view point of a long time 9mm Luger ammunition reloader (since 1978), and degreed mechanical engineer with a background in tolerance analysis and metrology (the science of measurement). I freely admit that very few of these ideas are mine. I have merely been graced with the time and aptitude to sift through thousands of articles on the subject, cull out the falsehoods, and thoroughly test the plausible. The results published here are the result of a nearly 15 year test period dealing almost exclusively with the 9x19 Luger cartridge, along with a smattering of 40S&W, 45ACP and 38 Super.

If this article succeeds, it is only because I am standing on the shoulders of giants. Thank you to all the giants of reloading who graciously shared their time and information.

• Taper crimp does not hold or help retain the bullet in the case mouth. In modern auto cartridges, the bullet is solely held in place by "case mouth tension", or in the parlance of bottle-neck rifle cartridges "case neck tension". In these instances, the case mouth internal diameter (after expansion) is usually 0.002" smaller than the bullet outside diameter. Thus when the bullet is installed, the elastic properties of the brass spring back around the bullet and help retain it in the case.

• If you can push your finished cartridge against your work bench with moderate pressure and have the bullet recede into the case, then you do not have a taper crimp issue. Your problem is: 1) your bullet OD is too small, or 2) your case mouth ID is too big. It is the Expander that sets the case mouth inside diameter. If the Expander is setting the case neck ID too large, then the bullet will not (can not) be held tightly.

• More taper crimp does not typically hold the bullet tighter. Most often, more taper crimp simply distorts the cartridge case to the extent that it buckles and actually loosens the grip on the bullet. However, results can also be dependent upon the brand of Crimp Die and brand of brass case.

• As outlined above, the cartridge case neck ID is typically smaller than the bullet OD, thereby constituting a "press fit" for the cartridge assembly. In order to physically fit a larger object into a smaller hole the case mouth entrance is "flared" or "belled" to make insertion easier. After the bullet is seated, this "belling" must be removed. Taper crimp is the name of the process that erases the flare, and returns the case mouth to its pre-flare shape.

Fig. A: Missing Taper Crimp on a Straight Walled Cartridge Case

• 45ACP, 38 Super, 380 Auto and 40S&W are examples of modern, American designed cartridges. As such the external diameters vary no more than .001-.002" over the entire length of the case, making these cartridges truly "straight walled" cases. If any flare or belling exists on the finished cartridge exterior, the cartridge simply refuses to enter the chamber. This makes incorrect taper crimp diagnosis on a true "straight walled" auto pistol cartridge quick and easy, as shown in Fig A.

Fig. B: SAAMI 9x19 Luger Cartridge Case Dimensions

However, the popular 9x19mm Luger cartridge is vastly different. It was designed in Germany way back in 1902. Look at the cartridge diagram in your reloading manual, or see Fig B. Unlike the previously mentioned auto pistol cartridges, the 9mm has a pronounced taper in its external shape, from 0.380 at the mouth to 0.391 at the rim. This means if there is any flare or belling on the finished round, the cartridge may go partially into the tapered chamber and stop, as in Fig C. Since the obstruction cannot be seen, diagnosis is much more difficult. Therefore, it is highly desirable to tackle proper taper crimp settings on the 9mm Luger during die setup and avoid these headaches.

Fig. C: Missing Taper Crimp on a 9mm Cartridge Case

• Owing to the tapered exterior of the 9mm series of cartridges (9x19, 9x21, 9x23, etc), when taking measurements of the taper crimp diameter, it is easy to confuse the increasing diameters of the case with the dimension of the taper crimp. Therefore, it is imperative that any measurements of taper crimp be made only on the last 1mm (.040") of case mouth, that part closest to the bullet. For this job I highly prefer a handheld caliper over a micrometer. See Fig D.

Fig. D: Measuring Taper Crimp on a 9mm Cartridge

Mathematically, the diameter of a proper taper crimp simply equals 1 bullet diameter plus 2 case wall thicknesses. Since taper crimp is only the process of erasing the belling, a more practical and direct way to discover the optimal dimension would be to simply press your bullet into a sized, but un-belled case and measure the outcome. On the 9mm with plated or jacketed bullets this measurement usually ends up somewhere between 0.375 and 0.378".

This is a great way if you intend to shoot one brand of bullet in one brand of brass. However, just as bullet diameters vary, case wall thickness between different brass makers also varies. If you shoot a variety of brass and/or bullets the total dimension will vary over a wider range, so another method is needed.

Reference the cartridge dimensions shown in your reloading manual. We could state that a cartridge with dimensions larger than those may never fit into your chamber. It also follows that a cartridge with dimension slightly smaller than those shown would always fit into your chamber. In my humble opinion then, we can draw the conclusion that the dimensions given in the SAAMI cartridge drawing are, for the most part, the maximum permissible cartridge dimensions.

Anyone who has ever fit a finely ground metal pin into a 0.0002" over-sized reamed hole will tell you that the pin is extremely hard to insert, it must be precisely aligned almost to the point of aggravation. The tight fit makes exact alignment mandatory. In much the same way, a cartridge for a slowly-loaded revolver can allow a tight fit. However in the case of an auto-loading automatic pistol, more clearance is required because the loading takes place in a fraction of a second. Even with a rounded bullet on the cartridge nose, in my experience an extra 0.002-0.003" of case clearance is needed to insure the lightning-fast feeding and complete slide closure ("going into battery") on an auto pistol.

Therefore the above observations spawn several thoughts.
• First, that a good and truly useful taper crimp diameter for any auto cartridge is going to be 0.002-0.003" less than the case mouth dimensions shown in the SAAMI drawings. That makes the useful taper crimp dimension for 9x19 Luger equal 0.380 minus 0.003, or in the range 0.376-0.378", with most people using 0.377-0.378". (Notice that these numbers also fit nicely with those from the "Optimal" section.)

• Secondly, taper crimp is a function of the barrel. This since correct taper crimp promotes good cartridge feeding, and feeding deals with the interface between the cartridge and barrel. Therefore, no matter what bullet style, weight, or diameter you might be loading, the taper crimp diameter will never change, unless you change your barrel.

• Because of the reasons above, if you see load instructions for a "light taper crimp" or "heavy taper crimp" you may want to assume that the writer is not thoroughly knowledgeable, and may wish to double check the rest of the load data.

Note: The following section does not apply to the Lee Factory Crimp Die. You are on your own when adjusting the FCD.

In order to properly set the Taper Crimp Die, it is best to make up about 5 to 8 "test cartridges" which have no primer and no powder. You'll need more than one test cartridge due to the material properties of the brass cartridge case. You'll also need the presence of a bullet, which acts as an anvil for the die to push against.

Set a test cartridge under the die and raise the ram all the way. Run the die down until it comes into full contact with the cartridge. Lower the ram. Lower the Taper Crimp Die another 1/4 turn and lock into position. Starting with the first test cartridge, crimp the cartridge and measure the taper crimp diameter. Keep lowering the die in very small increments (1/16 turns?) with a new test cartridge each time until the appropriate dimension is acquired. Use the test cartridges in order, and when you run out start again with cartridge number 1. Note that due to the "spring back" quality of brass, cases cannot change less than 0.003-0.004" in a single crimp. Therefore, do not try to achieve a 0.377" crimp diameter on a test cartridge initially measuring 0.379".

This procedure can take up to 30 minutes, but is well worth your patience. This tedious adjustment is also the best reason to use a separate Taper Crimp Die. By being a standalone die it never runs the risk of having its critical setting changed while modifying another cartridge parameter.

Of course, the final settings are always checked during production. Measure the first 15 cartridges of your next production run and make any final adjustments as required. After that, adjustments should only be needed on an annual basis to account for die wear. Remember too that taper crimp is for the benefit of the barrel; changing bullets does not change the taper crimp setting.

As a practical note, the barrel is always the ultimate arbitrator for any discussion of cartridge dimensions. Any time you wish to use the barrel as a gauge, the only safe method is to remove the barrel from the gun. To save time, you may find that an LE Wilson cartridge gauge closely mimics your chamber, but the barrel will always be the ultimate cartridge gauge.

Cartridges having the correct taper crimp have the ability to 1) fall into, 2) fully head space on the front lip of the chamber, and then 3) fall back out of the chamber using only their own weight. This test is a valid indicator for both taper crimp diameter and general ability to feed. Generally, a pleasing "Tink !" sound will result from a properly sized cartridge being dropped into the chamber. This is the result of the cartridge mouth seating on the end of the chamber.

I've used several different brands of Taper Crimp Dies over the years, but I keep coming back to the one-piece Lee die. For the 9mm Luger this is Lee product number 90780. Not only is it the least expensive die, I admire the ease of operation and the quality of the crimp. Midway USA currently list this die for around $14, however if you watch you can find it on sale for $10. This may be Lee's single best product.

Hope this helps.  ;)

Moderator Note

The information contained here is simply the best information available to one writer at the time. However, as in all things, "best" is usually a moving target. We always welcome your comments and constructive criticisms via PM. Thanks.
« Last Edit: May 05, 2018, 03:32:38 PM by Wobbly »
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