Understanding Ammunition Shape and Dimension Issues

[Wobbly]

Understanding Ammunition Shape and Dimension Issues

Buying ammunition for a CZ pistol can introduce several issues (especially in guns chambered for 9x19 Luger), which may present several hurdles for those new to CZ pistols. This document tries to educate CZ owners who do not reload on a number of ammunition issues and detail steps to help avoid common pitfalls. It's hoped that this format is easier to comprehend and reference, while at the same time being truly useful.


DOCUMENT FORMAT
This document tries to proceed in the most logical fashion so that readers at any level can easily follow along and comprehend the concepts. When the reader sees a red "asterisk" (*) at the beginning of any section, then the reader should recognize that this section contains a very important idea which the reader needs to fully understand before moving to the next section. If you need further clarification, then please don't hesitate to ask questions in the Ammo Questions and Handloading Techniques forum. This document is for members of the CZ Forum and asking questions is the only way moderators have to update the areas that may be unclear to the users.


NOMENCLATURE
*  Before we can begin, we need to clarify multiple terms referring to specific parts of the bullet, case and assembled cartridge so that we can all start on an equal footing. By starting with a clear understanding of the names we can erase years of slang, internet lore, hearsay and myth, while learning to use the technical language of cartridge construction. These terms are given in the first post of a separate document.  CLICK HERE

*  In this document we will use the term OAL to mean over-all cartridge length. This term is not standardized within the shooting hobby and other abbreviations (such as COL and COAL) are commonly used to denote the exact same measurement. By "OAL" we mean the total length of the assembled cartridge, from the flat base of the case head to the tip of the bullet's meplat.


HEADSPACING
*  Headspace is the design concept that controls cartridge insertion into the chamber. A feature has to be present to stop the cartridge from going too far into the chamber. If this feature is not present, then the firing pin would not be able to strike the primer. Every gun's chamber contains a feature that holds the cartridge securely in place (relative to the firing pin) so that it can be successfully fired. Revolvers and 22LR headspace on the oversized rim of the cartridge case. Bottleneck rifle cartridges headspace on the angled shoulder. Almost all auto pistol cartridges headspace on a step at the end of the chamber. The mouth of the auto pistol's cartridge case strikes this small ledge and is kept from moving further forward.



One of the other useful features of proper headspacing is that it also allows the "action" to fully close for the shooter's safety. If the chamber is not sealed off by proper headspacing, then hot gasses and molten shards of the brass case can be expelled in close proximity to the shooters face. In auto pistols, where there is a huge amount of variation in ammunition design, if the slide is not allowed to fully close the chamber, the gun can be said to fire Out Of Battery (OOB). This is an extremely dangerous condition since most modern handguns are using red-hot gasses to raise chamber pressures to the 30,000 psi range.

So understanding headspacing is far more than a mere academic exercise. The shooter's safety depends on it. In addition to the trauma sustained by the shooter, when using a polymer gun (such as a P07) the receiver can be totally destroyed by an OOB incident.




Look at this P07 carefully. The gun fired OOB. The unsupported brass case blew
out under immense chamber pressure. The escaping pressure destroyed this pistol.


BARREL DIMENSIONS
*  All barrels sold in the USA supposedly meet SAAMI specs, and yet no 2 manufacturers barrels are the same. Realize that each auto-pistol maker produces their barrel, chamber and freebore to slightly different dimensions from all other manufacturers. Those variations affect ammo feeding, types of ammo that the gun can accept, and to a great deal the accuracy obtained from shooting said ammo. When choosing ammo, the barrel's freebore length is of particular interest to the shooter. The following cartoon will try to graphically explain...



*  At the short end of the freebore length spectrum are the CZ pistols and a few other makes. These are made in eastern Europe where they still respect quality pistols and fine craftsmanship. One of the ways CZ achieves such incredible accuracy is by shortening the "freebore"; that part of the bore in front of the chamber that has no rifling. This section is also known as the "leade". The short freebore is not usually an issue with RN bullets (the only bullet type legal in Europe), and generally everyone is happy.

However when CZ's reach the USA they encounter numerous bullet styles they were never designed to chamber. When working with bullet types which are not sold in Europe, buyers need to be very careful. With conical bullets, like the Hornady XTP, notice that the major diameter, and resulting shoulder, is brought out well in front of the cartridge case mouth. Depending upon OAL, this shoulder can protrude enough to strike the end of the freebore. Additionally, RNFP and specialty SD ammo using JHP bullets may have a foreshortened ogive. The blunt-nose ogive common to this type bullet can strike the rifling. The graphic below shows how 3 different bullet shapes typically interface with the same CZ chamber.


How 3 different bullet shapes might interface with the same barrel

*  Therefore, the CZ's short freebore means that these auto pistols cannot shoot every cartridge offered by every ammo maker. The CZ owner simply has to be very selective with their ammo choices. Failure to do so can create a dangerous OOB condition.


PLUNK & SPIN TESTING
*  There are several ways to determine safe CZ cartridge lengths. However, the method described here quickly tells the owner that he's on safe ground without having to buy any tools. For these reasons members of the Ammo Forum have found the following method to be superior and we have standardized on this method. However, it does require that you take your naked barrel with you when you go ammo shopping.



Drop several sample rounds into the chamber, one by one. You should hear the barrel ring with a Ting! sound as each cartridge headspaces on the end of the chamber. It is important that these cartridges should be able to fall fully into and then exit the naked barrel's chamber using only their own weight. Secondly, since the end of the chamber is extremely smooth, the cartridge should be able to freely rotate back and forth over a quarter turn. This task should be very easy to complete, even with only the tiny bit of rim exposed above the end of the chamber. You will not know the exact extent of the clearance, but if 5 or 6 random rounds turn freely, then that box of ammunition should be OK in that pistol.

If the cartridges cannot be rotated, it may be a good idea to take a Sharpie marker and color the exposed bullet ogive. Then the finished round can be returned to the chamber and rotated back and forth about 20 degrees. After the round is withdrawn from the chamber, the marking will be scratched off anywhere the bullet is touching. The bullet's nose and ogive should never touch anything inside the chamber.

SUMMATION
*  All cartridge cases lengths are held to very tight tolerances. Therefore in factory ammunition, OOB conditions are almost ALWAYS caused by too much bullet protruding from the cartridge case. In other words, the cartridge is too long to use that OAL in that particular barrel. Buyers must always assume that ammunition that fails the Plunk & Spin test are prime candidates to cause an OOB occurrence in that particular gun.

*  The dangerous OOB condition is more prevalent in heavy bullets because heavy bullets are physically longer. Longer bullets then promote the tendency to use a longer cartridge OALs. In the 9mm Luger cartridge, this situation is most commonly found with 147gr bullets. Users should be extremely cautious of firing any ammo they have not personally tried in their CZ barrel using the Plunk & Spin test.


Hope this helps !