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February 17th, 2015
 Horizontal Wind-Drift vs. Distance
OK, here’s a challenge question for you.
Let’s see if you get it right.
Q: If the wind is blowing 10 mph from 9 o’clock and if my horizontal wind deflection is 0.7 inches at 100 yards, what is the horizontal drift at 1000 yards?
You may be thinking, “Well, since the target is ten times more distant, the wind-drift should be around 7 inches, maybe a little more since the bullet will be slowing down.” That sounds reasonable, right?
WRONG.
As you move from near to far, the increase in lateral deflection (from a 90° crosswind) is (roughly speaking) a function of the square of the multiple of distance. If your target is two times farther away, you use the square of two, namely four. If your target is five times farther away, you use the square of five, or twenty-five. In this example, the increased wind drift (from 100 to 1000 yards) is at least 0.7″ times (10 X 10) — over 70 inches (give or take a few inches depending on bullet type). We call that the Rule of the Square. This Rule lets you make a quick approximation of the windage correction needed at any yardage.
Precision Shooting and the Rule of the Square
I was going through some back issues of Precision Shooting Magazine and found many references to the Rule of the Square. This made me curious — I wondered how well the Rule really stacked up against modern ballistics programs. Accordingly, I ran some examples through the JBM Ballistics Trajectory Calculator, one of the best web-based ballistics programs. To my surprise, the Rule of the Square does a pretty good job of describing things.
EXAMPLE ONE — .308 Win (100 to 400 Yards)
For a 168gr Sierra MK (.308), leaving the muzzle at 2700 fps, the JBM-predicted values* are as follows, with a 10 mph, 9 o’clock crosswind (at sea level, 65° F, Litz G7 BC):
Drift at 100: 0.8 MOA (0.8″)
Drift at 200: 1.6 MOA (3.3″)
Drift at 400: 3.4 MOA (14.4″)
Here you can see how the Rule of the Square works. The rule says our drift at 200 yards should be about FOUR times the drift at 100. It the example above, 0.8″ times 4 is 3.2″, pretty darn close to the JBM prediction of 3.3″. Quoting Precision Shooting: “Note that the deflections at 100 yards are typically a quarter of those at 200; lateral deflections increase as the square of the range”. Precision Shooting, June 2000, p. 16.
EXAMPLE TWO — .284 Win (100 to 1000 Yards)
For a .284 Win load, with the slippery Berger 180gr Target Hybrids, the Rule of the Square still works. Here we’ll input a 2750 fps velocity, Litz G7 BC, 10 mph, 9 o’clock crosswind, (same 65° temp at sea level). With these variables, JBM predicts:
Drift at 100: 0.5 MOA (0.5″)
Drift at 500: 2.5 MOA (13.3″)
Drift at 1000: 5.9 MOA (61.3″)
Again, even with a higher BC bullet, at 1000 yards we end up with something reasonably close to the 100-yard deflection (i.e. 0.5″) multiplied by (10×10), i.e. 50 inches. The Rule of the Square alerts you to the fact that the effects of crosswinds are MUCH greater at very long range. In this example, our JBM-calculated drift at 1000 is 61.3″ — that’s over 100 times the 100-yard lateral drift, even though the distance has only increased 10 times.
Note that, even with a 5 mph 90° sidewind, the “Rule of the Square” still applies. The 1000-yard lateral deflection in inches is still over 100 times the lateral deflection at 100 yards.
Why This All Matters (Even in the Age of Smartphones)
 Now, some would say, “Why Should I Care About the Rule of the Square? My iPhone has a Ballistics App that does all my thinking for me”. Fair enough, but knowledge of this basic Rule of the Square enables a shooter to make an informed guess about necessary windage even without a come-up sheet, as long as he knows the distance AND can fire a sighter at 100 or 200 yards as a baseline.
For example, if I see empirically that I need 1″ windage correction at 100 yards, then I know that at 600 yards I need at least roughly (6 x 6 x 1″) or 36 total inches of drift correction, or 6 MOA. (To be precise, 1 MOA = 1.047″ at 100 yards). I can figure that out instantly, even without a ballistics chart, and even if my Smartphone’s battery is dead.
*Values shown are as displayed on the JBM-figured trajectory tables. The numbers can be slightly imprecise because JBM rounds off to one decimal place for both inches and MOA.
February 14th, 2015
 Many shooters are familiar with ballistics tables, weather programs, and even wind meters for smart devices, but few may know about a very handy Leveling Tool that comes factory-installed on Apple iPhones. The leveling function is a little-known option in Apple’s Compass App. It works well for a multitude of tasks.
There are a numerous reasons that a leveling tool should be in every rifleman’s range bag. From leveling optics during mounting to figuring out how much extra compensation is going to be required for a tricky angled shot, knowing just how far off things are from plumb can go a long way towards realizing success in the field.
This writer has used the leveling app on his iPhone to level a rifle on a rest while at the range. It definitely worked for “field expedient” leveling duties. That’s especially important for long-range applications. Just one degree of cant (tilt) can move your point of impact 7 inches at 1000 yards.
Of course, the iPhone level doesn’t use an actual bubble to find angles. Rather, it relies on the device’s sophisticated accelerometer to do so, and with a great degree of accuracy. Navigating to the level is done by first selecting the Compass App, at which point the device will need to be calibrated by rotating it a full 360 degrees. Once the compass is fully calibrated, simply make right swipe gesture to bring up the level — it will start operating immediately.
From there, use is intuitive and easy, like most iPhone Apps. Switching from horizontal plane to vertical is done by simply changing the physical axis of the phone. How do you know when you’ve got things just right — well the entire lower half of the screen turns green when everything is perfectly level. You’ll also see a zero° read-out, like this:
Bottom Line: If you already own an iPhone, you should definitely give this App a try. The price is right (free), and for a wide variety of tasks the iPhone Level App is actually pretty handy.
February 7th, 2015
The Applied Ballistics team tested six (6) same-length/same-contour Bartlein barrels to observe how twist rate might affect muzzle velocity. This unique, multi-barrel test is featured in the book Modern Advancements in Long Range Shooting. That book includes many other fascinating field tests, including a comprehensive chronograph comparison.
Barrel Twist Rate vs. Velocity — What Tests Reveal
by Bryan Litz
When considering barrel twist rates, it’s a common belief that faster twist rates will reduce muzzle velocity. The thinking is that the faster twist rate will resist forward motion of the bullet and slow it down. There are anecdotal accounts of this, such as when someone replaces a barrel of one brand/twist with a different brand and twist and observes a different muzzle velocity. But how do you know the twist rate is what affected muzzle velocity and not the barrel finish, or bore/groove dimensions? Did you use the same chronograph to measure velocity from both barrels? Do you really trust your chronograph?
Savage Test Rifle with Six Bartlein Barrels
Most shooters don’t have access to the equipment required to fully explore questions like this. These are exactly the kinds of things we examine in the book Modern Advancements in Long Range Shooting. In that book, we present experiments conducted in the Applied Ballistics lab. Some of those experiments took on a “Myth Buster” tone as we sought to confirm (or deny) popular pre-conceptions. For example, here’s how we approached the question of barrel twist and muzzle velocity.
Six .308 Win Barrels from Bartlein — All Shot from the Same Rifle
We acquired six (6) barrels from the same manufacturer (Bartlein), all the same length and contour, and all chambered with the same reamer (SAAMI spec .308 Winchester). All these barrels were fitted to the same Savage Precision Target action, and fired from the same stock, and bench set-up. Common ammo was fired from all six barrels having different twist rates and rifling configurations. In this way, we’re truly able to compare what effect the actual twist rate has on muzzle velocity with a reasonable degree of confidence.
Prior to live fire testing, we explored the theoretical basis of the project, doing the physics. In this case, an energy balance is presented which predicts how much velocity you should expect to lose for a bullet that’s got a little more rotational energy from the faster twist. In the case of the .30 caliber 175 grain bullets, the math predicts a loss of 1.25 fps per inch-unit of barrel twist (e.g. a 1:8″ twist is predicted to be 1.25 fps slower than a 1:9″ twist).
Above, data shows relationship between Twist Rate and Muzzle Velocity (MV) for various barrel twist rates and rifling types. From fast to slow, the three 1:10″ twist barrels are: 5R (canted land), 5 Groove, 5 Groove left-hand twist.
We proceeded with the testing in all 6 barrels from 1:8” to 1:12”. After all the smoke cleared, we found that muzzle velocity correlates to twist rate at the rate of approximately 1.33 fps per inch of twist. In other words, your velocity is reduced by about 5 fps if you go from a 1:12” twist to a 1:8” twist. [Editor: That’s a surprising number — much less than most folks would predict.] In this case the math prediction was pretty close, and we have to remember that there’s always uncertainty in the live fire results. Uncertainty is always considered in terms of what conclusions the results can actually support with confidence.
 This is just a brief synopsis of a single test case. The coverage of twist rates in Modern Advancements in Long-Range Shooting is more detailed, with multiple live fire tests. Results are extrapolated for other calibers and bullet weights. Needless to say, the question of “how twist rate affects muzzle velocity” is fully answered.
Other chapters in the book’s twist rate section include:
· Stability and Drag – Supersonic
· Stability and Drag – Transonic
· Spin Rate Decay
· Effect of Twist rate on Precision
Other sections of the book include: Modern Rifles, Scopes, and Bullets as well as Advancements in Predictive Modeling. This book is sold through the Applied Ballistics online store. Modern Advancements in Long Range Shooting is also available in eBook format in the Amazon Kindle store.
February 4th, 2015
Sierra Bullets Product Development Manager Mark Walker recently acquired a barrel with canted lands. It turns out he needed to modify his bore-cleaning methods. His brushes and patches were not following the rifling… and he was well on his way to ruining his barrel before he figured out the solution. Read about Mark’s interesting (and puzzling) experience. This article first appeared in the Sierra Bullets Blog.
 Lessons Learned (The Canted Land Mystery)
by Mark Walker
Sometimes when you have done something so often, you take for granted that it will work for all equipment. In this case, I had used my cleaning process and equipment for years with no problems however a new barrel on a rifle caused me to rethink how I clean.
Last year, the barrel on my mid-range benchrest rifle decided to give up the ghost. After doing some research and asking fellow shooters, I decided to purchase a barrel that had rifling with “canted” lands. The barrel arrived and after looking it over, everything looked great.
Threading and chambering went very well with the barrel indicating in very straight and cutting very smoothly. After torqueing the barrel to the action, I went about loading some ammunition to break the barrel in with. At the range, the barrel shot as good as any I have ever had. Even with loads that were thrown together with no tuning whatsoever.
 During the break in, I would clean the barrel after every five shots or so just because that’s what everyone says to do. The barrel cleaned up extremely well, however I noticed that the first patches down a dirty barrel would cause the cleaning rod to vibrate as I pushed them down the tube. That was something that I had never experienced before so I was a little concerned….
After looking at the barrel with a bore scope, everything looked clean and no indication of what might have caused the vibration in the rod. I did notice some surface marks in the bore that traveled perpendicular with the bore. Usually when a barrel is lapped, all marks follow the rifling twist so these marks parallel to the bore where another phenomenon that I had never seen before. The mystery was getting deeper.
After another range session where the barrel shot lights out, I brought it home to clean it as before. The first patches down the barrel again caused the strange vibration in the rod. After stopping and thinking about the vibration and the strange marks in the bore, I checked the bearings in cleaning rod handle to make sure it was spinning freely and everything seemed to be in working order. I then decided to mark the cleaning rod to make sure it was actually turning when it went down the barrel. Bingo — this revealed the problem.
When the first patch went down the barrel and the rod didn’t even attempt to turn, the light bulb went on. The patches and even the bronze brushes were simply skipping over the tops of the rifling and not following the rifling at all. I tried tighter patches and larger brushes, but the only thing that seemed to fix the problem was pushing them down the bore as slowly as possible while watching the mark on the rod to make sure it was turning. Had I continued to clean as I normally do, I surely would have ruined the barrel!
Once I figured out the problem, the barrel shot great and my cleaning process worked just like every other barrel I have ever owned except for having to go slow with the rod. This just goes to show that even though you may have done something a thousand times before, you should always be aware of what your equipment is telling you.
February 1st, 2015
 In our Shooters’ Forum, there was an discussion about a range that was threatened with closure because rifle over-shoots were hitting a farm building over two miles from the firing line. One reader was skeptical of this, asking “how’s that possible — were these guys aiming at the stars?” Actually, you may be surprised. It doesn’t take much up-angle on a rifle to have a bullet land miles down-range. That’s why it’s so important that hunters and target shooters always orient their barrels in a safe direction (and angle). Shooters may not realize how much a small tilt of the barrel (above horizontal) can alter a bullet’s trajectory.
How many degrees of muzzle elevation do you think it would take to hit a barn at 3000 yards? Ten Degrees? Twenty Degrees? Actually the answer is much less — for a typical hunting cartridge, five to seven degrees of up-angle on the rifle is enough to create a trajectory that will have your bullet impacting at 3000 yards — that’s 1.7 miles away!
Five degrees isn’t much at all. Look at the diagram below. The angle actually displayed for the up-tilted rifle is a true 5.07 degrees (above horizontal). Using JBM Ballistics, we calculated 5.07° as the angle that would produce a 3000-yard impact with a 185gr .30-caliber bullet launched at 2850 fps MV. That would be a moderate “book load” for a .300 Win Mag deer rifle.
Here’s how we derived the angle value. Using Litz-derived BCs for a 185gr Berger Hunting VLD launched at 2850 fps, the drop at 3000 yards is 304.1 MOA (Minutes of Angle), assuming a 100-yard zero. This was calculated using a G7 BC with the JBM Ballistics Program. There are 60 MOA for each 1 degree of Angle. Thus, 304.1 MOA equals 5.068 degrees. So, that means that if you tilt up your muzzle just slightly over five degrees, your 185gr bullet (2850 fps MV) will impact 3000 yards down-range.
Figuring Trajectories with Different Bullets and MVs
If the bullet travels slower, or if you shoot a bullet with a lower BC, the angle elevation required for a 3000-yard impact goes up, but the principle is the same. Let’s say you have a 168gr HPBT MatchKing launched at 2750 fps MV from a .308 Winchester. (That’s a typical tactical load.) With a 100-yard zero, the total drop is 440.1 MOA, or 7.335 degrees. That’s more up-tilt than our example above, but seven degrees is still not that much, when you consider how a rifle might be handled during a negligent discharge. Think about a hunter getting into position for a prone shot. If careless, he could easily touch off the trigger with a muzzle up-angle of 10 degrees or more. Even when shooting from the bench, there is the possibility of discharging a rifle before the gun is leveled, sending the shot over the berm and, potentially, thousands of yards down-range.
Hopefully this article has shown folks that a very small amount of barrel elevation can make a huge difference in your bullet’s trajectory, and where it eventually lands. Nobody wants to put holes in a distant neighbor’s house, or worse yet, have the shot cause injury. Let’s go back to our original example of a 185gr bullet with a MV of 2850 fps. According to JBM, this projectile will still be traveling 687 fps at 3000 yards, with 193.7 ft/lbs of retained energy at that distance. That’s more than enough energy to be deadly.
January 25th, 2015
In the video below, Forum member Eric Cortina shows how to install a Jewell Benchrest trigger into a Kelbly F-Class Panda action. You could follow the same simple procedure to install a Jewell in a standard Panda action. Kelbly’s sell both standard and long versions of the F-Class Panda action. Both versions feature integral recoil lugs in the front.
To see more detail in this “how-to” video, you can zoom it to full-screen size. Simply click the full-screen icon (4-cornered frame) just to the right of the YouTube logo in the lower right.
January 21st, 2015
Forum member Preacher recently crafted a nice varmint rifle for fellow Forum member Dave 0. (aka “Waskawood”). But rather than buy an off-the-shelf stock, Preacher crafted this stock all by hand, starting from a laminated blank panel. He calls this stock project his “Axe Job”.
CLICK for Full-size Photo
This stock is being used on a prairie dog rifle, chambered for a 17-caliber wildcat, the 17 VHA, which is based on an H&K 4.6x30mm parent case. With about nine grains of 300 MP pistol powder, the 17 VHA drives 20-grainers at about 3850 fps. (SEE details at end of article).
The ‘Axe Job’
Report by Preacher
I like carving with the laminates because all the lines are right there in front of my eyes, so it’s easy to follow along and get it just right, until it’s pleasing to the eye. I never use a template, I just keep checking the lines as I go along. I have all the needed equipment to power build one of these, but I really enjoy the time spent on the hand work. From start to completely ready-to-install, I’ll have about six (6) weeks into one of these stock projects. A lot of that is drying time for the clear coats.
A little work with the hand axe, after a trip through the band saw…
The majority of the laminated blank panels I use for my gunstocks are purchased directly from Cousineau Wood Products or from Rutply.com. You have to buy at least four full panels at a time, all the same color, but that will yield eight (8) stocks. Seems like I have a little over $150.00 in a blank large enough to start making a full-sized, benchrest-style stock.
A little work with a chisel…
A little work with a rasp. (Before I was rich and famous and could afford really good rasps, I used a good old horse shoe rasp.)
A little more work with the chisel…
Preacher’s Advice on Carving Your Own Stock
The one main advantage of being older that dirt, and tormented with MS the past 40 years, is lots of free time to enjoy what ever I can do these days, as long as I can set down to do it, and I can make a lot of wood chips setting down.
Any one can do this if they have the time to devote to it. All it takes is time and a good eye for details. I made a lot of firewood over the years, until I got the hang of it. Most all those problems were inletting, and screw hole spacing. Get those right the first time and you’re on your way….
A little more work with the rasp…
A few coats of Auto clear has it about buttoned up…
Micro 17 VHA Wildcat
 Here’s the finished rifle built by Preacher for Dave, using the ‘Axe Job’ stock. Dave tells us: “Preacher chambered the rifle for the 17 VHA, a wildcat based on the H&K 4.6x30mm MP7 PDW case necked down to 17 caliber. There are numerous articles in the Varmint Hunter’s Magazine about it. This efficient little round shoots 20gr ballistic tips at 3850+ fps. That’s not too shabby for ‘nine point something’ grains of pistol powder.”
“My intentions for my 17 VHA rifle are to plop down in the middle of a PD town with my swivel bench and shoot prairie dogs. I also thought it would be a nice platform to test the accuracy of the cartridge. If I like the little round as well as I think, I plan to build a more practical rifle that I can carry. I really want to thank Preacher for his patience with me through this project, as it was my first custom build.”
January 13th, 2015
Forum member Erik Cortina recently launched his own YouTube Channel dedicated to precision reloading and accurizing. Erik’s videos demonstrate the proper use of specialized reloading tools and provide helpful hints. Erik’s latest video is about the “mother of all brass trimmers”, the Giraud powered case trimmer. Erik says: “It you do volume reloading… this is the only trimmer to get. It not only trims to length but it also chamfers your case mouth inside and out.” In his video, Erik offers some very clever and useful tips that will help you get the most from your Giraud.
The Giraud trimmer is very precise. When set up correctly, it can trim brass with amazing consistency. In the video, Erik trims 5 pieces of brass in 15 seconds (6:32 mark). He then measures all five with precision calipers (7:00-8:08). All lengths are exact within .0005 (half a thousandth). Erik notes that the Giraud trimmer indexes off the case shoulder. As long as you have fire-formed brass with consistent base-to-shoulder dimensions, you should get very consistent trim lengths.
The secret to the system is a 3-way cutting head. This cutter can be swapped in and out in a couple minutes with wrenches provided with the kit. Erik has three different heads; one each for 6.5mm, 7mm, and .30 caliber. The video shows how to adjust the cutting heads to match caliber diameter (and to get the desired amount of inside/outside chamfer).
This is a manufacturer’s photo showing an older model.
To trim and chamfer cases, you simply insert them nose-first into the cartridge-specific case-holder. Erick offers a smart tip — He uses a die locking ring to position the cartridge holder (3:15). This can be locked in place. Erik says die locking rings work much better than the hex-nuts provided by Giraud (with the hex-nut, one must re-set cut length each time you change case-holders.)
The Giraud can be used in either horizontal or vertical modes. Erik prefers to have the trimmer aligned vertically, allowing him to push cases down on the trimmer head. But the trimming unit has twin sets of rubber feet, allowing horizontal or vertical orientation.
Improved Case-Holder Made with Chamber Reamer:
For his .284 Shehane, Erik had to create his own case-holder (Giraud does not make one for that wildcat cartridge). Erik used his chamber reamer. To his surprise, Erik found that the brass was easier to trim in the custom case holder (compared to the Giraud-made spring-loaded holders). With a perfect fit, trimming and case extraction went more smoothly and the process was easier on his hands. (See 9:00-10:00). Based on Erik’s experience, you may want to create your own custom case-holder.
Trim Bullet Meplats Also
With a special bullet-holder fitting and meplat cutter head, the Giraud power trimmer can be used to trim bullet meplats. Trimming meplats can help make the Ballistic Coefficents of a batch of bullets more consistent. Uniforming meplats is also often done as a first step in the process of “tipping” bullets to improve BC.
January 13th, 2015
 Are you re-barreling a match rifle and need to know if you will still make weight? Or perhaps you want to select the right contour to hit an optimal carry weight for a new varmint rifle? Dan Lilja offers FREE software that will calculate barrel weight for straight contour, straight taper, and radius-tapered barrels. Dan’s software even calculates how fluting alters barrel weight.
For general info on barrel weight calculation for straight and straight tapers, read this article on Lilja’s website. Click HERE for another article explaining weight calculation with barrels that have a radiused (curved) contour section.
Here are the free software programs offered by Dan Lilja. Right click and “Save As”:
Straight Contour (.exe file) | Straight Contour (Excel file) | Contoured Barrels with Radius (Excel file)
December 29th, 2014
 In our Shooters’ Forum a reader asked: “How much neck tension should I use?” This prompted a Forum discussion in which other Forum members recommended a specific number based on their experience, such as .001″, .002″, or .003″. These numbers, as commonly used, correspond to the difference between case-neck OD after sizing and the neck OD of a loaded round, with bullet in place. In other words, the numbers refer to the nominal amount of interference fit (after sizing).
While these commonly-used “tension numbers” (of .001″, .002″ etc.) can be useful as starting points, neck tension is actually a fairly complex subject. The actual amount of “grip” on the bullet is a function of many factors, of which neck-OD reduction during sizing is just one. Understanding these many factors will help you maintain consistent neck tension as your brass “evolves” over the course of multiple reloadings.
Neck Tension (i.e. Grip on Bullets) Is a Complex Phenomenon
While we certainly have considerable control over neck tension by using tighter or looser bushings (with smaller or bigger Inside Diameters), bushing size is only one factor at work. It’s important to understand the multiple factors that can increase or decrease the resistance to bullet release. Think in terms of overall brass-on-bullet “grip” instead of just bushing size.
One needs to understand that bushing size isn’t the beginning and end of neck tension questions, because, even if bushing size is held constant, the amount of bullet “grip” can change dramatically as the condition of your brass changes. Bullet “grip” can also change if you alter your seating depth significantly, and it can even change if you ultrasonically clean your cases.
Bullet grip is affected by many things, such as:
- 1. Neck-wall thickness.
- 2. Amount of bearing surface (shank) in the neck.
- 3. Surface condition inside of neck (residual carbon can act as a lubricant; ultrasonic cleaning makes necks “grabby”).
- 4. Length of neck (e.g. 6BR neck vs. 6BRX).
- 5. Whether or not the bullets have an anti-friction coating.
- 6. The springiness of the brass (which is related to degree of work-hardening; number of firings etc.)
- 7. The bullet jacket material.
- 8. The outside diameter of the bullet and whether it has a pressure ridge.
- 9. The time duration between bullet seating and actual firing (necks can stiffen with time).
- 10. How often the brass is annealed
— and there are others…
Seating Depth Changes Can Increase or Decrease Grip on Bullet
You can do this simple experiment. Seat a boat-tail bullet in your sized neck with .150″ of bearing surface (shank) in the neck. Now remove the bullet with an impact hammer. Next, take another identical bullet and seat it with .300″ of bearing surface in another sized case (same bushing size/same nominal tension). You’ll find the deeper-seated bullet is gripped much harder.
 Neck-Wall Thickness is Important Too
I have also found that thinner necks, particularly the very thin necks used by many PPC shooters, require more sizing to give equivalent “grip”. Again, do your own experiment. Seat a bullet in a case turned to .008″ neckwall thickness and sized down .003″. Now compare that to a case with .014″ neckwall thickness and sized down .0015″. You may find that the bullet in the thin necks actually pulls out easier, though it supposedly has more “neck tension”, if one were to consider bushing size alone.
In practical terms, because thick necks are less elastic than very thin necks, when you turn necks you may need to run tighter bushings to maintain the same amount of actual grip on the bullets (as compared to no-turn brass). Consequently, I suspect the guys using .0015″ “tension” on no-turn brass may be a lot closer to the guys using .003″ “tension” on turned necks than either group may realize.
Toward a Better Definition of Neck Tension
As a convenient short-cut, we tend to describe neck tension by bushing size alone. When a guy says, “I run .002 neck tension”, that normally means he is using a die/bushing that sizes the necks .002″ smaller than a loaded round. Well we know something about his post-sizing neck OD, but do we really have a reliable idea about how much force is required to release his bullets? Maybe not… This use of the term “neck tension” when we are really only describing the amount of neck diameter reduction with a die/bushing is really kind of incomplete.
My point here is that it is overly simplistic to ask, “should I load with .001 tension or .003?” In reality, an .001″ reduction (after springback) on a thick neck might provide MORE “grip” on a deep-seated bullet than an .003″ reduction on a very thin-walled neck holding a bullet with minimal bearing surface in the neck. Bushing ID is something we can easily measure and verify. We use bushing size as a descriptor of neck tension because it is convenient and because the other important factors are hard to quantify. But those factors shouldn’t be ignored if you want to maintain consistent neck tension for optimal accuracy.
Consistency and accuracy — that’s really what this all about isn’t it? We want to find the best neck tension for accuracy, and then maintain that amount of grip-on-bullet over time. To do that you need to look not only at your bushing size, but also at how your brass has changed (work-hardened) with time, and whether other variables (such as the amount of carbon in the neck) have changed. Ultimately, optimal neck tension must be ascertained experimentally. You have to go out and test empirically to see what works, in YOUR rifle, with YOUR bullets and YOUR brass. And you may have to change the nominal tension setting (i.e. bushing size) as your brass work-hardens or IF YOU CHANGE SEATING DEPTHS.
Remember that bushing size alone does not tell us all we need to know about the neck’s true “holding power” on a bullet, or the energy required for bullet release. True bullet grip is a more complicated phenomenon, one that is affected by numerous factors, some of which are very hard to quantify.
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