January 04, 2011
One case prep step that really does make a difference.
By Charles E. Petty
There are a lot of variables that affect the accuracy of reloads, and there's a lot of advice floating around out there on how to do it "right." Aside from the safety aspects, I don't heed most of it, but there is one aspect of case prep I do pay attention to--something we've learned from the benchrest crowd--and that is the importance of case necks.
As our bullet begins its journey from chamber to target, the first thing it has to do is exit the case mouth. If the brass is thicker on one side than the other, as the neck expands from pressure buildup the thick side is going to contact the chamber first and this will, in turn, push the base of the bullet toward the thin side. The net result is that the bullet is going to start off with a slight bias to one side and not enter the leade (throat) straight on.
Advertisement
Benchresters' chambers are cut to specific dimensions for the neck and then they turn the brass to fit, and turning case necks so the brass is an even thickness all around is one solution. Most new brass is going to vary a little in neck thickness, and that generally doesn't matter, but when you are working with a really accurate rifle and trying to maximize performance it can become an issue.
Neck turning works, but it's tedious, lengthy process that becomes a major chore if you need a lot of cases. In the past I've just bitten the bullet and turned them but a recent discovery has provided another approach. I've known about neck measuring instruments but never really given them their due until I saw a new one from Redding. It uses a dial indicator to easily show variation in case necks. How thick they are isn't as important as how much they vary. In its instructions, Redding says that variation should be less than 0.0015 inch for best results.
Advertisement
Redding's gauge uses a dial indicator that is graduated in thousandths of an inch, but the marks are far enough apart so that it is easy to estimate another half. There is a stand that holds the indicator. A small mandrel holds a caliber-specific pilot and centers itself through the flash hole on the other end.
All you have to do is put an empty case on the mandrel and adjust the pilot so the indicator contacts the neck roughly in the middle. Then adjust the indicator to zero and rotate the case through a full circle. Any variation in the neck thickness will show up as a wobble on the indicator dial.
The scientist in me likes precise measurements, and my lazy side prefers sorting cases to turning necks. And it is easy enough to find out whether this is beneficial. If we can, through a simple measuring tool, avoid much of the busy work of case prep, there has to be good in there somewhere.
I have a Cooper Phoenix in .223 topped with a new Leupold 8.5-25X that I have been working with for some time and have used in a series of tests to find a good load in terms of components and loading practice. Most of my initial testing has been done with new Winchester brass, and there was some left, so I sorted it into three separate groups of 15 rounds each. The first had neck thickness that varied no more than 0.001, the second was around 0.0015 and the third was over 0.002 variation.
Now I know that 15 rounds does not a statistical certainty make, but it's enough to give you a good indication if something's working. For each batch I fired two fouling shots and then followed with three record groups. Then I cleaned the gun and allowed it to cool to ambient temperature before firing the next batch.
My range is known for unpredictable winds, so I have wind flags up, and for this particular test I waited for one of those nice rare days where winds were very light and steady. All loads are new Winchester brass using Remington small rifle benchrest primers and charged with 24.0 grains of Alliant 10-X with a Hornady 52 grain A-max bullet. Overall length is 1.889 inches. The results are shown in the accompanying table.
Before we even look at the accuracy, pay some attention to the extreme spread and standard deviation. Those numbers represent all the shots fired under each neck condition. Conventional wisdom seems to hold that you must have a small standard deviation to get good accuracy, but time and again I've seen that simply to be untrue. Sure it's nice to have one, but that in no way promises better accuracy.
The accuracy results show the typical influence of the bell curve, but the averages give a strong indication that variations in neck thickness make a difference. And the appealing thing is that this is a quick and dirty sorting procedure. I didn't do any of the other case prep stuff to primer pockets and flash holes. All I did was sort by variation in neck thickness.
Obviously we will find differences in other makes of brass, and my choice of Winchester was driven by the fact that it is readily available and reasonably priced. The other good news is that the Redding gauge doesn't require much work on our part and doesn't cost a bunch--retail is $96.
The main thing, at least for me, is that it is clear to see the benefit for the effort involved. I surely won't do it for every cartridge I load, but for my varmint and tactical rifles where accuracy is a major concern it can easily become part of the routine.
table#specialTable {padding: none;background: black; font-family: Arial,Helvetica, sans-serif;text-align: left;font-size: 11px;}table#specialTable tr {color: red; background:blue;}#specialTable td {margin: 2px; padding: 2px; background: #cccccc; color:black;border-right: 1px solid black; border-bottom: 1px solid black;}#specialTable td.noright {margin: 2px; padding: 2px; background: #adadad;color: black;border-right: none;border-left: none; border-bottom: 1px solidblack;}#specialTable td.noleft {margin: 2px; padding: 2px; background: #adadad;color: black;border-right: 1px solid black;border-left: none; border-bottom:1px solid black;}#specialTable td.noborder {margin: 2px; padding: 2px; background: #669966;color: black ;border-right: none; border-bottom: 1px solid black;}#specialTable th {margin: 2px; padding: 2px; background: #ffffff; color:black;font-family: Verdana, Arial, Helvetica, sans-serif;text-align:left;font-size: 9px;}#specialTable tr.offcolor td { background: #ffffff;}#specialTable tr.footer {}#specialTable td.footer {}#specialTable h3 { margin: 0; padding:0;font-size: 15px; border-bottom: 2pxsolid #669966;}.bottom { 2px solid black;}
EFFECTS OF CASE NECK VARIATION Neck Variation (in.) Muzzle Velocity (fps) Extreme Spread (fps)
Standard Deviation Group 1 Size (in.) Group 2 Size (in.) Group 3 Size (in.) Avg. Group (in.) <0.001 3,358 114 29 .457 .243 .550 .417 0.0015 3,351 73 24 .514 .652 .493 .553 >0.002 3,353 54 16 .628 .768 .674 .690 Groups are five shots 100 yd. from benchrest. Velocity is the average of 15 shots measured at 15 feet with a PACT chronograph.
