Case Prepping for Accuracy
May 17, 2011
I recently undertook a task at the request of component supplier Kaltron Outdoors.
Case prep is crucial for optimum accuracy. Cleaning and uniforming the primer pocket is one step the author always performs when handloading; it ensures proper fitting of the primer.
I recently undertook a task at the request of component supplier Kaltron Outdoors. The firm imports Lapua bullets and cartridge cases and VihtaVuori propellants from Finland. Product manager Jeannie Bolda asked me to determine just how many times Lapua brass could be reloaded compared to other major brands. Since I had a Ruger Mark II Target model rifle, I chose to test .308 Winchester match handloads.
Instead of simply reloading and shooting until the last batch of cases was left standing, I intended to gather data to trend case length growth, primer pocket looseness, the effects of cartridge overall length/propellant charge weight increments on accuracy, etc. I've completed a dozen cycles so far, and only one of the six batches that I began with has failed. I'll surely be reporting on the results in a future column, but for now, let me share the various steps I performed to select and prep the cartridge cases for building the most accurate handloads.
First, I thoroughly cleaned my Ruger MKII rifle (again). I packed it and my hoard of ammo to the range and fired two, five-shot groups with each of six different brands of factory ammo to fireform the brass. By fireforming the brass in the actual test rifle, I intended to eliminate a factor that could affect the subsequent test data.
Back at my workshop, I prepared an Excel spreadsheet and recorded the velocity and accuracy data. After tumbling the cases in treated corncob media, I examined them carefully to find any defects. There were none.
Next, I popped the primers out using Lee's handy decapping die, uniformed the primer pockets, and weighed each case. I selected five of each headstamp that weighed the closest to each other as possible.
In order to analyze and make rational conclusions, I recorded specific attributes to establish a detailed baseline of the dimensional characteristics, e.g., as-fired neck diameter and case length, neck wall thickness, etc., of each batch of brass and added the data to the spreadsheet. Only then did I continue to prepare the 30 test cases for reloading.
I have a vintage RCBS competition die set and adjusted the full-length sizer die so that it resized about 90 percent of the neck. Because the cases were fireformed in the test rifle's chamber, I wanted to avoid bumping the shoulder back, i.e., maintain the near-optimum headspace. With this setup, the sizer die only partially reshapes the case body and leaves a small portion of the as-fired neck diameter at the shoulder intact.
|Hornady Concentricity Tool|
There are many lubricant choices today compared to when I started reloading years ago. The sticky lube and lube pad have long since been replaced on my bench by small tins of Hornady and Redding lubricating wax. I use a case neck brush with polymer bristles to scrub and lube the inside of each case neck and apply more lube--sparingly--to the case body with my fingers. The case neck requires only a touch because as you size the case in a full-length sizer die, lube flows forward to the shoulder. If there's too much, it can be trapped and hydraulically dent the case. Little dents are unsightly, but the case may be salvageable. Large dents compromise the structural integrity, and the case is scrap.
When you're finished resizing, it's time to clean up the mess. I usually use an old towel or tee shirt and vigorously wipe the lube off each case. I also use a pistol cleaning rod fitted with a slotted jag and cotton patch to swab the lube and powder residue out of the case neck. This step helps improve the consistency of the neck tension on the bullet. I often dump the cases back into the tumbler (with untreated corncob media) to clean and polish them thoroughly. If you choose to include this step, make sure you check the primer pockets afterwards and remove the bits of media that become lodged in the flash holes.
The chamber dimensions of production rifles like my Ruger comply with SAAMI standard (read, generous) tolerances. So it's not cut tight to the minimum dimensions. My initial measurements indicate the case necks typically expanded 0.010 inch. By only partially resizing the case neck, each handload will, theoretically, center itself in the chamber with the bolt fully closed. The bullet should be (again, theoretically) precisely aligned with the bore and therefore enhance the prospects for shooting a good group.
For the .308, the maximum case length (after sizing) is 2.015 inches. The recommended trim-to length is 2.005 inches. After trimming, you should slightly chamfer the case mouths inside and out and deburr/uniform the flash holes. Again, for production rifles, there's really no need to turn the case necks. If the case-neck thickness varies no more than one and a half thousandths of an inch, neck tension should be adequately consistent and not adversely affect accuracy. If it varies more than three thousandths, it's not a good candidate for an "accuracy" case, so set it aside.
A couple of the 30 test cases exhibited two thousandths variation in neck thickness, and several others measured one and a half thousandths. The rest held to even tighter tolerances. I concluded they were all good enough and dispensed with neck turning.