Of all of the items that affect rifle accuracy perhaps none are as important and have as much affect as the area surrounding the neck and the mouth of the cartridge case. You might have the best rifle in the world but if you ignore the requirements of the cartridge in relation to seating depth and the tension on the projectile it may destroy any potential accuracy. What on the surface appears to be inconsequential is in fact a veritable minefield of misinformation when we talk about accuracy. All items are interconnected, change one and you alter the rest, the good news is that all of the issues are in fact under the control of the reloader.
The very first consideration in relation to cartridge case tension relates to just who made the cases you are about to use. True all case will have the same external dimensions; they have to in conformance with SAAMI standards, thicker necks may grip the cases tighter.
On the beginning we have to make decisions about projectile type, powder to propel it and ensure that the selected projectile is compatible with the barrel twist of the rifle. In addition the type of projectile may have another further affect depending on whether it is flat based or has some form of boat-tail. A singular question invariably opens up another multitude of others. Modern technology has also affected such decisions simply because projectiles are no longer a jacket filled with a lead core, in this day and age projectiles may be made of pure copper or another metal thus affecting the relationship between the projectile and the case neck.
To ensure that the best accuracy is obtainable from any particular rifle/ projectile combination the reloader must ensure that the twist of the barrel will stabilize the chosen projectile. Sometimes a particular projectile is literally on the edge of stability, in the author’s experience it is better to choose another more stable projectile which will over time be more consistent.
It is merely a function of projectile length and velocity, the necessary calculations can be accessed via the Berger website, and some computer programs will also cover the calculations. In round figures the boat tailed projectile will require a twist rate approx. one inch faster than is flat based cousin for the same weight, at high projectile weights in relation to calibre this may in fact be the deciding factor if the barrel will stabilize a particular projectile
At this particular point in time it is possible to measure the amount of “grip” imposed on the shank of the projectile by the case mouth. Ballistic Tools (in USA) makes a number of Case Mouth & Neck Tension gauges for popular calibres. In the case of .22 calibre mouths the gauge is cut on .224/.221/.218 and .215 inch diameters. It is immediately apparent what a particular case mouth size is either via a normal expander button or one reduced in size by a selected bushing.
The author sorts cases according to size using these gauges and suggests that it is better than no measurement at all. We mentioned the amount of grip on the projectiles shank.
A simple statement covering a wide range of possibilities, all or any of which will both alter just how a particular case will shoot, it is in fact a very complex subject, ignore it at your peril! The starting point has to be the thickness of the case neck, this will vary between makers, there is liable to be some variation, best measured with a tubing micrometre. So question number one is, do we necessarily want/need to eliminate and/or reduce this variation?
In the authors opinion whilst there are many issues that affect case mouth tension and/or grip the one that appears to have the most affect is annealing or lack of it. Work hardening of the brass case occurs at every stage of use, if you do not do anything about it eventually the case neck has no grip at all and the new projectile will simply slide into the case without any resistance. The way to prevent this is by annealing which restores the ductile properties of the brass back to its original state. The author would suggest that where accuracy is a requirement cases should be annealed after every firing, this may be considered by some as extreme.
The use of straight line dies or dies with internal bushings to reduce the size of case necks by a specific amount may require some experimentation.
You should be aware that annealing and/or ultrasonic cleaning of cases will remove the carbon residue from cases which provides a form of lubrication on the insides of necks when seating another projectile; this may be replaced by dry lubricant (graphite) if considered necessary. Normally overall cartridge length is controlled by the magazine and is this specified in reloading manuals.
Another problem occurs when we start altering seating depth is the relationship between the neck of the case and the amount of the projectile within the neck, either more or less, thus the relationship will change the amount of grip imposed.
With some projectiles there is little difference, a recent experience with a .243 Ackley cartridge was enlightening. The projectiles had the same nose profile and weight at 103 grains. The boat-tailed version was shot first and proved to be very accurate with good velocity, but I did wonder if a flat based version would be much different. The short story is that that was nothing between them in terms of overall accuracy.
At this stage we also need to mention that aside from any other considerations the cartridge needs to be concentric with the axis of the bore. Thus seating of the projectile is paramount. Most accuracy related cartridges will use straight line dies for both seating and resizing, in short they are simply more accurate overall.