Check ya neck!By Matthew Cameron
- 19th Apr, 2020 Apr 19, 2020, 10:08 AM
- 4 Comments
Of all the items that contribute to rifle accuracy perhaps the most important three are projectile seating depth, case neck runout and case neck tension.
The latter is the most complicated, it's a complex subject with many variables. With a consistent neck tension we will have a consistent velocity, all other items being equal.
It is well known that if you take new brass and continue to reload it over time the metallic structure of the brass work-hardens and the ability of the neck to “grip” the new projectile becomes less.
Perhaps less well known are the other considerations that affect the neck's ability to “grip” the projectile, including the type of projectile, its length, and the metallic make-up of the jacket.
All the items above will have some effect on the amount of pressure applied to the projectile. From an accuracy point of view the trick is to have consistent pressure applied to any given projectile.
There is obviously a combination that a particular rifle prefers above all others and will produce the best accuracy in combination with the right amount of powder and a chosen primer. Perhaps you could say you're looking for a combination that your barrel is “comfortable” with.
“Perhaps less well known are the other considerations that affect the neck's ability to “grip” the projectile, including the type of projectile, its length, and the metallic make-up of the jacket.”
In the normal course of reloading the case mouth is resized either within a normal recapping and sizing die, or by reducing the size of the case neck in a bush of a specific size. But the question remains - just how do you quantify the amount of tension?
About twelve months ago I came across an interesting set of reloading tools made by Ballisictools. They make a number of gauges that actually measure with some precision the inner size of the case neck.
The .270/6mm gauge is graduated from .277 inches and reduces in three steps of .003 inches each. The steps are marked with lines around the circumference of the shaft.
In similar fashion the .243 gauge measures .243” and has three steps each .003 of lesser size. This makes the actual size of each case mouth quickly evident. In addition the company makes two go/no go gauges for both small and large primer pockets.
The gauges may also indicate whether any particular case was annealed correctly - perhaps a particular case has a metallic structure that is not within specifications.
I have found several cases that refused to be sized after annealing. I pulled these from the bunch but did not dump them, I find they do have other uses on the reloading bench.