Written by: Kim Lockhart, Copyright.
Rifle Cartridge Case / Brass:
The cartridge case is the one component in reloading that is so generally passed over when looking for accuracy, when in fact it is one of the most critical for accuracy. It is the cartridge case that has to hold the bullet in perfect alignment with the rifles bore, any deviation here and accuracy will suffer.
The first rule when it comes to cartridge cases is never mix cases from different manufacturers or even same manufacturer with different lot numbers or batch run. If you really are looking for accuracy loads, bulk brass is cheap so when ordering order a minimum of 100 cartridge cases before standardizing cases.
Weighing Cartridge Cases:
The first step in standardizing cartridge cases is weighing each and every one and then separating them by weight, separate in 1 grain increments or less.
If there is even 1/10 of a grain difference in weight between the cases that means there is a difference in the internal dimension and strength structure of the case. This will change stress expansion areas of the case which in turn will change the way the cartridge headspace's in the rifle chamber when it is fired.
Each time the cases are loaded and fired the heavier cases will stretch less between the cases shoulder and head than the lighter ones. Evidence of this can be noted by measuring case length after firing the cartridges, they simply do not stretch the same and this does affect accuracy.
The cartridge case capacity is directly affected which is nothing more than the volume of the cartridge case but of which in turn affects velocity hi-lo spread and will affect bullet impact or accuracy.
Case Neck Thickness:
The one case dimension that really helps shrink group spread is uniform neck thickness. Ideally, necks shouldn't vary more than .0015" in thickness. Before measuring, bulk brass should be run through with an expander ball to remove dents (resizing die with expander ball). All cartridge cases should be checked for this condition after every third loading because brass flows forward and will thicken the cartridge cases neck.
How the completed cartridge fits in the chamber and throat of the rifle barrel affects accuracy.
A quick, way to determine several important case and cartridge dimensions is by using the
RCBS CaseMaster Gauging Tool,
case neck concentricity,
case neck thickness,
case length and