October 20, 2020
Achieving half-minute-of-angle accuracy with an accurate rifle can come fairly easy with nothing more than standard reloading tools and off-the-shelf components, along with a bit of bullet jump and powder-charge juggling. But once that level of accuracy is reached, improvements in accuracy usually come slowly, most often in very small increments. Some things we do when precision-loading ammunition will prove to be more important than others, but each one can contribute to minor decreases in group size.
Some rifles respond nicely to some things we do, and others don’t. But we will never know until we try. And while accuracy improvements in a precision-built custom rifle can often be predicted fairly closely, the same does not hold true for many mass-produced rifles. We must also be realistic. While a few tenths of an inch improvement in accuracy and a 25-fps reduction in velocity spread can be very important to a 1,000-yard benchrest shooter, they are less important to the hunter who refuses to shoot at game beyond ethical distances. Due to their smaller targets, varmint shooters probably benefit more from precision handloading than big-game hunters.
Preparing cases is time-consuming, but it can pay off. There are two primary goals. The first goal is to arrive at a batch of cases that when chambered, consistently hold the axis of a bullet as close as possible to perfect alignment with the axis of the bore of the barrel. The other goal is to minimize shot-to-shot variation in velocity. A maximum variation of 20 fps is good, but 10 fps is better.
When you’re rounding up a batch of cases, make sure they are all unfired and made by the same manufacturer. Even better if the same lot number is on all packaging. A close visual inspection will eliminate any cases with slightly off-center flash holes or other imperfections.
Most short-distance benchrest competitors shoot the 6mm PPC formed by necking up and fire-forming Lapua .220 Russian brass. I get the impression that sorting cases by weight is not as popular there as among those who shoot to 1,000 yards and beyond. Whether sorting is beneficial has long been debated, but as popular thinking goes, a heavier case has less capacity than a lighter case of the same caliber due to more material retained during the forming process. In other words, between two .308 Winchester cases, the one with the slightly thicker wall and/or web will have less capacity, and when the same charge-weight of powder is fired in the two cases, pressure and velocity will be a bit higher with the heavier case.
Is the capacity difference enough to matter? I believe the answer to that depends on the quality of the cases used, but considering how quickly weight-sorting can be accomplished with a good digital scale, I totally understand why its proponents insist on doing it. Some sort cases into small groups weighing exactly the same and keep them that way throughout their service life. Others sort them into batches with weight within each batch varying a slight amount.
Sorting cases by weight is not an exact science simply because, contrary to popular belief, the capacities of two cases weighing exactly the same can differ a bit in water capacity. Even when the web and the wall thickness of cases are exactly the same, there can be slight variations in the amount of material in their rims. This is why some competitive shooters sort cases by weighing them after they have been fired and then weighing them again after they are filled to the brim with water. (For obvious reasons this is done with the fired primer still in the case.)
Some shooters run the fired cases through a sonic cleaner and then through a digital dryer prior to weighing them. Others don’t. Either way, after the cartridges are capacity-sorted with water, they are placed in the dryer.
The next step is to full-length resize with a standard, no-bushing die. Trimming to the exact same length is important at this point because case neck length determines the cutter travel distance of an outside neck-turning tool. Because the mouth of the case serves as a stop for the tool, all necks must be of the exact same length. How much the wall of the neck should be thinned is determined by the chamber neck diameter of the barrel.
The barrels of rifles used by 100- and 200-yard benchrest shooters have chamber necks small enough to require considerable neck wall thinning. A popular chamber neck diameter for the 6mm PPC is 0.262 inch, although some are a bit smaller or larger. The usual cartridge neck diameter with a 0.243-inch bullet seated is 0.260 inch for a 0.262-inch chamber throat. Having 0.001 inch of space all around the neck of the cartridge gives it room to expand enough to release its grip on the bullet during firing. This means that for a 0.262-inch chamber, case neck wall thickness is reduced to 0.0085 inch. The RCBS Vernier ball micrometer I use for measuring neck-wall thickness is capable of accuracy within 0.0001 inch.
While such small chamber necks are popular among short-distance benchrest shooters, they are seldom seen in other types of rifles. But precision-built target rifles put together for shooting at great distances often have chamber necks of smaller diameter than the typical factory-built rifle. Among other things, the closer chamber neck diameter is to cartridge neck diameter, the less brass is worked during firing and resizing. When rounding up Bartlein barrels to be chambered to 6mm Creedmoor and 6.5 Creedmoor for a 17-pound switch-barrel rifle, I wanted chamber neck diameters to be 0.002 inch or so larger than Lapua brass neck-turned just enough to uniform wall thickness. That turned out to be 0.2728 inch for the 6mm Creedmoor cases and 0.2927 inch for the 6.5mm Creedmoor cases. So I ordered chamber reamers that cut neck diameters of 0.275 inch for the former and 0.295 inch for the latter.
Uniforming neck-wall thickness accomplishes two things: It closely aligns the axes of the bullet and the case, and equally important, it makes neck tension on bullets closer to the same for the entire batch of cases, which goes a long way toward keeping shot-to-shot velocity spread low.
When performing this operation for a factory-built rifle with SAAMI chamber dimensions, your goal is to keep carefully adjusting the cutter blade of the turning tool until material is being removed from the thick section of the neck and the blade is lightly touching but not actually removing brass from the thin section. As a result, a minimal amount of material is removed, and the wall of the neck will be the same thickness all the way around.
There was a time when this was not practical for factory rifles because when the neck wall of a case was thinned, full-length resizing dies were sometimes incapable of reducing neck diameter enough for adequate tension on a seated bullet. That problem vanished with the introduction of bushing-style dies by Wilson, RCBS, Redding, Hornady, Forster, and others. Use a digital caliper or a micrometer to measure the diameter of a case with a bullet seated and then choose a sizing bushing 0.002 inch smaller. In other words, if cartridge neck diameter is 0.268 inch, you would need a bushing with an interior diameter of 0.266 inch.
Velocity spread can sometimes be reduced by trying different amounts of case neck tension on the bullet, and I would go a step further by also having 0.267-, 0.265-, and 0.264-inch bushings on hand. While a bushing only 0.001 inch smaller than cartridge neck diameter is sometimes used when loading match ammo, more case neck tension should be applied to ammo that will be used for hunting.
Several bench-mounted tools are available. Hornady has the Neck Turn Tool, while Lyman, RCBS, and Forster offer turning attachments for their case-trim tools. Small hand tools are available from RCBS, K&M Precision, Sinclair International, and Forster. I bought one from barrelmaker Paul Marquart during the 1970s, and I am still using it.
Through the years I have accumulated several because having one for each rifle eliminates the task of resetting the cutter. Having two for the same rifle is not a bad idea if a considerable amount of brass has to be removed. For my benchrest rifle in 6mm PPC with a 0.262-inch chamber neck, one tool is permanently set for the first rough cut, and another is set for the finishing cut.
Hand tools can be used with a small clamp-style case holder, but for large quantities of cases, power is better. A cordless drill or screwdriver capable of running in the 400- to 500-RPM range works fine. The 0.250-inch hex shaft of the Sinclair #05-200 driver works with either, and it accepts Sinclair case holders available for cartridges of various rim diameters. Simply clamp the power tool into a padded bench vise, press the switch, and shave away brass.
Hand tools come in all sizes, and in addition to being more comfortable in the hand, a larger model like the Sinclair NT-4000 does not heat up as quickly when a power tool is used. Its cutter depth and micrometer-style cutter travel adjustments are accurate to 0.0001 inch.
A snug but not overly tight fit between the inside of the case neck and the mandrel of the turning tool is important. This is achieved by running each case through a standard full-length resizing die with its expander button removed, and then through a Sinclair expander die. Its 7/8-14 threads work with about any reloading press.
The mandrel of the expander die should be 0.0005 to 0.001 inch larger than the mandrel of the turner. Applying a small dab of sizing wax to both mandrels between cases prevents binding. A carbide mandrel in the turning tool is better than stainless steel. If the tool becomes warm to the touch, take a break and allow it to cool off.
Case feed and removal speed are important. Go too fast and spiraled marks will be left on the neck of the case. The finished surface should be quite smooth. With a case still spinning, give its neck a final light polish with 0000-grade steel wool.
Adjusting the tool to cut slightly into the shoulder of the case at its juncture with the neck is important. If the cut is correct, you should see a tiny shoulder at the beginning of the neck. A magnifying glass or eye loupe magnifier helps. But don’t overdo it because if the cut is too deep, the case may separate there during firing.
If the cut is not slightly into the shoulder, a ring, or “donut,” will appear inside the case at its shoulder/neck juncture after the case is fire-formed. It can usually be seen by using a flashlight to peer inside the neck of the case. It can also be felt by pushing a flat-based bullet far enough into the neck of a fired case for its base to reach the shoulder/neck juncture.
If overall cartridge length is such that the base of a seated bullet is positioned forward of the donut, don’t worry about it. If bullets are seated deeper than that, the donut should be removed with a custom reamer sized to remove it without removing material from the inside surface of the case neck.
As mentioned earlier, to ensure the exact same cut depth into the shoulders of all cases, they should be trimmed to the same length prior to neck-turning. If you are uneasy about ending up with a bunch of cases with the dreaded donut affliction, make the cut into the shoulders of three cases and load and fire them twice in your rifle. Then take a look inside the necks of the cases.
Uniforming Primer Pockets
I once shot an extremely accurate prototype rifle in .223 Remington with a firing pin that could be adjusted to vary the amount of energy it delivered to a primer. When it was adjusted to deliver the same impact for each shot, groups measured less than a half-inch. When the firing pin was adjusted to deliver lighter and heavier impacts between each shot, accuracy went to pot.
That should have come as no surprise. Years before I had read an article written by Sinclair International founder Fred Sinclair, who was also a top-ranked benchrest shooter at the time. According to his tests, primer seating depth variations can vary firing pin strike energy by as much as 20 percent. This can cause variations in primer flame intensity, and that can cause variations in propellant ignition that can increase velocity spread. This is why uniforming primer pockets is an important step of precision handloading.
As a case is formed at the factory, the bottom of its primer pocket is slightly rounded, while the bottoms of primers are flat. A small carbide cutter is used to flatten the bottom of the primer pocket and cut it to uniform depth. It can be done by hand, but the shank of the cutter will have 8-32 threads and removing it from its handle and attaching it to one of the stations on a power case-prep center makes the job both faster and easier. A stop shoulder on the tool prevents it from cutting the primer pocket too deeply.
Deburring Flash Holes
I prefer to perform the final step by hand. The punch used to form the flash hole of a case at the factory sometimes leaves a small burr inside the case that can constrict primer flame travel. Lyman, RCBS, Hornady, Forster, Sinclair, and other companies offer this type of tool. The burr is not as likely to appear in Lapua cases, but I give them the same treatment anyway because the tool cuts a shallow bevel on the inside of the flash hole. When purchasing this tool, keep in mind that the flash holes of some cases made by Lapua are smaller than the American standard and require a smaller-diameter cutter. Those that spring to mind are 6.5x47 Lapua, 6mm Creedmoor, 6.5 Creedmoor, and .220 Russian. Redding offers deburring tools for both flash hole diameters, and an adjustable stop controls the cutting depth of the tool.
With a batch of prepared cases now in hand, in the next, and final, installment of this series I will take a close look at loading them for the best possible accuracy.
To read Part 1 of this series, The Accurate Rifle, click here.