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When Accuracy Counts: Neck Turning
See Part One at:
Turned necks promote concentric release of the bullet squarely into the rifling. That sounds nice and, let’s face it, very accuracy inspiring, but understanding what it does is more important than being able to say it in highbrow gun-speak. The reason neck turning promotes better accuracy is tied to how a bullet is introduced into the bore. To understand this interaction, the handloader must look at the interaction between the case neck, which is aiming the bullet into the rifling, the release of the bullet from the case and how the bullet first engages the rifling.
Case necks as received from their makers are not of uniform thickness. They exhibit low and high spots on the brass as well as different thicknesses when measured at different points on the neck using a ball micrometer. When fired, the neck expands to release the bullet, but may not do so cleanly if the expansion isn’t uniform on all sides of the bullet. Turning reduces neck thickness variations and allow the bullet to be release squarely into the rifling. Think of this as the first target your bullet needs to hit precisely on the road to accuracy. When it is perfectly released it strikes the center of the bore as it swages into the rifling. Anything less will adversely affect its flight.
Some custom made chambers require cartridges with turned necks. Typically they will show another number engraved after the cartridge identification on the barrel. So a rifle stamped 6mm PPC USA .265 NK is sharing one extra bit of information: the chamber neck is .265” in diameter. Cartridges loaded for this chamber can have an outside neck diameter no larger than .262” to .2635” to properly release a bullet into the bore. If you ever buy used custom rifles, or inherit one, make sure to check the barrel for custom dimensions. Rifles with chambers that require turned necks will produce higher than anticipated pressure with cartridges that use standard SAAMI neck dimensions.
Wildcatters or shooters who reduce cartridge neck diameters on parent cases, like necking a .308 Win down to a .243 Win, will sometimes need to turn necks. Some cases, like the .300 Blackout, are trimmed substantially before the neck is re-formed using brass from the case body. See story on forming .300 Blackouts at: http://blog.westernpowders.com/2015/03/making-300-aac-blackouts/ Because the case body brass is usually thicker, it can leave the outside neck dimension too large to release the bullet cleanly. In either case, knowing the neck dimension of your chamber and comparing it to your loaded cartridge dimension can solve a lot of hassles when dealing with unexpected pressure signs. Always make sure that loaded ammunition has an outside neck diameter at least .002” to .003” smaller than the chamber’s neck dimension.
The easiest way to check your chamber neck dimension is to measure a case immediately after it is fired. Adding .001 to this measurement, to account for brass spring-back, will give you a good approximation of your chamber’s neck dimension. A good rule for standard chambers is to allow .003” difference between the loaded cartridge neck diameter and the chamber neck dimension for clean bullet release. A chamber with a .251” neck required loaded cartridges with .248” loaded neck dimension or smaller for clean bullet release.
Turning Necks for No-Turn Chambers
It seems counter-intuitive to consider turning case necks for rifles that use standard neck dimension chambers, but the practice is commonly done by shooters seeking more accurate handloads. Neck turning for this type of chamber involves removing the high points on the neck, rather than turning for a pre-determined outside dimension. Necks turned for trueness will show where high spots have been cut away, leaving darker, lower areas untouched. When fired the brass will expand, ironing out the highs and lows into one consistent exterior surface against the chamber walls.
Case Trimming Sets the Neck Length
Uniforming the case neck length on all of the brass to be turned promotes consistency in the lathe’s cut, especially at the junction of the neck and shoulder which can be a problem point on turned cases. Trimming also creates a square case mouth, perpendicular to the bullet shank, which allows the bullet to be seated squarely. For the sake of simplicity, start with the suggested “Trim-To” length for your cartridge. Don’t feel that this number is set in stone. The only requirement is that all of the cases have the same length neck.
Preparing Brass for Neck Turning
Brass that is to be turned needs to fit on the turning tool’s mandrel tightly enough to cut accurately and loosely enough to allow for easy insertion and turning. There are two schools of thought on this when it comes to the relationship between the lathe’s mandrel dimension and the interior neck dimension. Some
turning tools require the use of a precision-cut expanding plug to open the case mouth. The Forster tool used in this story uses a turning mandrel about .001″ smaller than the expanding ball used in the match sizing die removing the need for a second sizing step. Whichever system is used, the case mouth needs to slightly larger than the turning tool’s mandrel, with the case mouth being between .0005 and .001 larger.
Setting Neck Turner Length-of-Cut
The neck-to-shoulder junction is the critical point in neck turning. Set too long and the cutter will undermine the shoulder allowing the neck to break off in the chamber during firing. Setting the cut distance short of the
shoulder leads to an accuracy harming phenomena called the “Dreaded Donut.” The donut forms when the outside of the case expands to fit the chamber, pushing back the untrimmed portion of the neck inside the case. It shows up as a raised ring inside the case neck which will adversely affect bullet
release. When properly set, the cutter should run the length of the neck and very slightly undercut the shoulder. Like so many things that mix art with science, the difference between just right and too much has to be learned by practice. Once the length of cut is set, set it in place and move on to setting neck thickness.
Setting Depth of Cut
On no-turn chambers, the goal is only to remove the high spots. With a cartridge on the mandrel screw the cutter down until it lightly touches the neck. Remove the case and advance the cutter slightly further inward. The Forster guide suggests one half of one of the graduations marked on the cutter knob. Each mark represents .001″ of cutter movement.
For best consistency, holding the tool body seems to work better than locking it in a vise. A cartridge handle provides mechanical advantage, although there are several adaptors on the market that allow cordless screwdrivers to soak up some the muscle work. The case is rotated clockwise throughout the cutting process.
The cut must be made slowly and steadily. Progressing too quickly will induce a spiraling cut rather than a smooth one that thoroughly covers the case neck. After the first cut has been completed, the cutter is again slightly advanced and the process is repeated until about 75% – 80% of the case shows bright areas that have been touched by the cutter. With the highs removed, the case will expand symmetrically in the chamber.
With chambers that require fixed outside neck diameters, turning is a bit more demanding. The easiest way to create precisely turned necks for custom chambers is to us some simple mathematics. Take the chamber neck dimension minus .002″ and subtract it from bullet diameter. That will give you the maximum neck wall thickness. That number divided by two will give you the finished neck wall thickness. Here is the math on a 6mm PPC using a .265” chamber neck dimension:
.265 Neck – .002 = .263 .263 – .243 = .020 .022/2 = .010 Neck thickness
Some handloaders use a feeler gage to set the cutter to this depth make the cut in a single pass with then another to account for cutter spring back. Others make two cuts, first making a roughing cut and then a finer finishing cut. Both work. The bottom line is that a loaded cartridge needs to measure at least .002″ smaller than the chamber neck dimension. Methods vary, but good results win matches.
Neck Turning Tips
Turning the neck twice without adjusting the cutter will usually produce a smoother cut than one single pass.
Cases need to turn freely on the mandrel. Sizing wax on the mandrel helps.
When you measure the case mouth thickness, remember, what you do on one side you do on the other. A cut that removed .001” from the neck wall thickness reduced the overall diameter of the neck by .002”.