When neck-turning cases, it’s a good idea to extend the cut slightly below the neck-shoulder junction. This helps keep neck tension more uniform after repeated firings, by preventing a build-up of brass where the neck meets the shoulder. One of our Forum members, Craig from Ireland, a self-declared “neck-turning novice”, was having some problems turning brass for his 20 Tactical cases. He was correctly attempting to continue the cut slightly past the neck-shoulder junction, but he was concerned that brass was being removed too far down the shoulder.
Craig writes: “Everywhere I have read about neck turning, [it says] you need to cut slightly into the neck/shoulder junction to stop doughnutting. I completely understand this but I cant seem to get my neck-turning tool set-up to just touch the neck/shoulder junction. It either just doesn’t touch [the shoulder] or cuts nearly the whole shoulder and that just looks very messy. No matter how I adjust the mandrel to set how far down the neck it cuts, it either doesn’t touch it or it cuts far too much. I think it may relate to the bevel on the cutter in my neck-turning tool…”
Looking at Craig’s pictures, we’d agree that he didn’t need to cut so far down into the shoulder. There is a simple solution for this situation. Craig is using a neck-turning tool with a rather shallow cutter bevel angle. This 20-degree angle is set up as “universal geometry” that will work with any shoulder angle. Unfortunately, as you work the cutter down the neck, a shallow angled-cutter tip such as this will remove brass fairly far down. You only want to extend the cut about 1/32 of an inch past the neck-shoulder junction. This is enough to eliminate brass build-up at the base of the neck that can cause doughnuts to form.
The answer here is simply to use a cutter tip with a wider angle — 30 to 40 degrees. The cutter for the K&M neck-turning tool (above) has a shorter bevel that better matches a 30° shoulder. There is also a 40° tip available. PMA Tool and 21st Century Shooting also offer carbide cutters with a variety of bevel angles to exactly match your case shoulder angle*. WalkerTexasRanger reports: “I went to a 40-degree cutter head just to address this same issue, and I have been much happier with the results. The 40-degree heads are available from Sinclair Int’l for $15 or so.” Forum Member CBonner concurs: “I had the same problem with my 7WSM… The 40-degree cutter was the answer.” Below is Sinclair’s 40° Cutter for its NT-series neck-turning tools. Item NT3140, this 40° Cutter sells for $14.99. For the same price, Sinclair also sells the conventional 30° Cutter, item NT3100.
Al Nyhus has another clever solution: “The best way I’ve found to get around this problem is to get an extra shell holder and face it off .020-.025 and then run the cases into the sizing die. This will push the shoulder back .020-.025. Then you neck turn down to the ‘new’ neck/shoulder junction and simply stop there. Fireforming the cases by seating the bullets hard into the lands will blow the shoulder forward and the extra neck length you turned by having the shoulder set back will now be blended perfectly into the shoulder. The results are a case that perfectly fits the chamber and zero donuts.”
* 21st Century sells carbide cutters in: 15, 17, 20, 21.5, 23, 25, 28, 30, 35, 40, 46 and 50 degrees. PMA Tool sells carbide cutters in: 20, 23, 30, and 40 degrees, plus other angles by special order.
The U.S. Army Marksmanship Unit regularly published a reloading “how-to” article on the USAMU Facebook page. One excellent “Handloading Hump Day” post covered preparation of once-fired 5.56x45mm brass. This article, the first in a 3-part series, has many useful tips. If you shoot a rifle chambered in .223 Rem or 5.56x45mm, this article is worth reading. You can obtain once-fired Lake City 5.56x45mm brass for less than half the cost of premium .223 Rem brass.
This week, Handloading Hump-Day will answer a special request from several competitive shooters who asked about procedures for morphing once-fired GI 5.56mm brass into accurate match brass for NRA High Power Rifle use. The USAMU has used virgin Lake City (LC) 5.56 brass to win National Championships and set National Records for many years. In this 3-part series, we’ll share techniques proven to wring match-winning accuracy from combat-grade brass.
GI brass has an excellent attribute, worth noting — it is virtually indestructible. Due to its NATO-spec hardness, the primer pockets last much longer than most commercial brass when using loads at appropriate pressures.
Preparing Once-Fired GI 5.56 Brass for Reloading (Part 1 of 3)
Assuming our readers will be getting brass once-fired as received from surplus dealers, the following steps can help process the low-cost raw material into reliably accurate components.
1. Clean the Brass
First, clean the brass of any dirt/mud/debris, if applicable. Depending on the brass’s condition, washing it in a soap solution followed by a thorough rinsing may help. [This step also extends the life of the tumbling media.] Approaches range from low-tech, using gallon jugs 1/2 full of water/dish soap plus brass and shaking vigorously, to more high-tech, expensive and time-consuming methods.
2. Wet-Tumbling Options (Be Sure to Dry the Brass)
When applying the final cleaning/polish, some use tumblers with liquid cleaning media and stainless steel pins for a brilliant shine inside and out, while others take the traditional vibratory tumbler/ground media approach. Degree of case shine is purely personal preference, but the key issue is simple cleanliness to avoid scratching ones’ dies.
If a liquid cleaner is used, be SURE to dry the cases thoroughly to preclude corrosion inside. One method is to dump the wet brass into an old pillow case, then tilt it left/right so the cases re-orient themselves while shifting from corner to corner. Several repetitions, pausing at each corner until water stops draining, will remove most water. They can then be left to air-dry on a towel, or can be dried in a warm (150° F-200° F max) oven for a few minutes to speed evaporation.
Shown below are Lake City cases after cleaning with Stainless Media (STM). Note: STM Case cleaning was done by a third party, not the USAMU, which does not endorse any particular cleaning method.
NOTE: The USAMU Handloading (HL) Shop does not RE-load fired 5.56 brass. We use virgin LC brass with our chosen primer already staked in place. However, our staff has extensive personal experience reloading GI brass for competition, which will supplement the Shop’s customary steps. In handloading, as in life, there are many ways to accomplish any given task. Our suggestions are note presented as the “only way,” by any means. Time for loading/practicing is always at a premium. Readers who have more efficient, alternative methods that maintain top accuracy are invited to share them here.
3. Inspect Every Case
Once dry, inspect each case for significant deformation (i.e., someone stepped on it), damaged mouths/necks and case head/rim damage. Some rifles’ ejectors actually dig small chunks of brass out of the case head — obviously, not ideal for precision shooting. Similarly, some extractors can bend the case rims so badly that distortion is visible when spinning them in one’s fingers. These can be used for plinking, but our match brass should have straight, undamaged rims.
Dented case mouths are common, and these can easily be rounded using a conical, tapered tool, [such as a .223 expander mandrel. A dummy 7.62 or .30-06 cartridge with a FMJ spitzer can also work.] If most of your brass is of one headstamp, this is a good time to cull out any odd cases.
4. Check the Primers Before Decapping
Your clean, dry and inspected brass is now ready for full-length sizing, decapping and re-priming. Historically, primer crimps on GI brass have caused some head-scratching (and vile language) among handloaders. Our next installment will detail efficient, easy and practical methods to remove primer crimp, plus other useful handloading tips. Until next week, Good Shooting!
Accuracy Potential of Mil-Surp 5.56×45 Brass
So, how accurate can previously-fired GI surplus brass be in a good National Match AR-15? Well, here’s a data point from many years ago that might be of interest. A High Power shooter who wrote for the late Precision Shooting magazine took a Bill Wylde-built AR match rifle to a registered Benchrest match. He had no difficulty obtaining consistent 0.5-0.6 MOA accuracy at 200 yards using LC brass and a generic “practice” load that was not tuned to his rifle.
The Tactical Rifle Shooters Facebook Group recently showcased tools used to measure case headspace before and after “bumping” the shoulder. After a case is fired, hand-loaders who full-length size their cases will typically bump the shoulders back anywhere from .001″ to .0035″, depending on the rifle and application. With our 6mmBR and Dasher cases we like about .0015″ bump.
You want the amount of case sizing and bump to be the same for all your brass. To ensure uniformity, it makes sense to measure your cases before and after the FL sizing process. When we have time, we check every case. Other folks will simply check the first 3-4 cases coming out of the FL sizing die to ensure the FL die setting is correct and delivering desired headspace/bump.
1. Whidden Gunworks Shoulder Bump Gauge
There are a variety of tools that can be used to measure shoulder bump. Our favorite is a special cartridge-specific bushing made by Whidden Gunworks. The Whidden Shoulder Bump Gauge enables you to adjust your sizing die to the desired measurement. The bump gauge is attached to your calipers with a set screw and determines the measurement from the base to the shoulder of the case. The photo below, from Tactical Rifle Shooters, shows the Whidden Bump Gauge for the .375 CheyTac cartridge.
2. Dave Manson Vertical Comparator with Dial Read-Out
Dave Manson states: “This tool was designed to make life easier for the advanced shooter and re-loader by allowing precise measurement of ammunition, case, and chamber headspace. With this information, the re-loader will be able to fine-tune clearances and fits between his ammunition and chamber, with resultant improvements in accuracy and case life.” The functions of the Manson Comparator are:
1. Measure headspace of factory or reloaded ammunition
2. Quantify chamber headspace by measuring headspace of a fired case
3. Ensure minimal shoulder set-back when setting up re-loading dies
4. Compare base-to-ogive length to ensure consistent bullet-to-rifling relationship.
In addition to the Dial Indicator and Stand, the $130.00 Vertical Comparator is supplied with multiple Datum Blocks of precise length and inside diameter (.3300″/.3750″/.4000″/.4375″). MORE INFO HERE — Catalog page 20.
3. Hornady L-N-L Headspace Comparator System
Hornady’s Lock-N-Load Headspace Comparator system is easy-to-use and handy. You can get a kit with Red bushing-holder body and 5 bushings for $43.43 at Midsouth. Hornady explains: “The Lock-N-Load® Headspace Comparator… gauge measures variations in brass before and after firing or re-sizing. It allows for headspace comparison between fire-formed brass and re-sized brass.” IMPORTANT: Hornady states: “To determine the proper bushing diameter for your cartridge, simply add the neck diameter and the shoulder diameter and divide that number by two. Use the bushing closest to that number.” Hornady offers five: .330″, .350″, .375″, .400″, and .420″.
One tip — We have found the Hornady gauges may vary a little from unit to unit even with the same nominal size. If you have more than one gauge for the same cartridge, test each on your brass — you may then note a slight difference in your bump measurements. There is also an Anvil Base Kit that mounts to the opposite blade on the caliper. This provides a more stable surface for the base of your case.
4. L.E. Wilson Case Gage Depth Micrometer
If you are looking for precise “bump” measurements without having to mess with calipers and clamp-on gauge blocks, you may want to consider the L.E. Wilson Case Gage Depth Micrometer. This takes very precise, repeatable measurements, but you need to know your starting point. The manufacturer explains: “Every reloader should know exactly how much your Full Length Sizing Die is pushing back the shoulder. With the NEW Case Gage Depth Micrometer you can do just that! It has never been easier to measure you cases headspace before and after sizing. The Depth Mic allows you to slip the micrometer perfectly over the top of the Gage with your case inserted into the Gage and take a measurement. Micrometer has graduations of .001″. The Case Gage Depth Micrometer is set to a zero of .100″ on the scale at our factory. Because of differences in ‘feel’ and temperature, we include a the Gage Block for you to test Zero and to adjust if necessary.”
5. Pistol Brass Case DIY Bump Gauge
Last is a “field expedient” set-up if you do not have any of the comparator tools shown above. A sized .45 ACP case (or other suitable pistol case) can be used to measure shoulder bump. The mouth of the pistol case sits on the shoulder of your rifle cartridge brass.
Make sure the .45 ACP case is trimmed square and that it is round. We recommend you first run it through an expander, then size it, trim it and chamfer. Next, take the .45 ACP case and slip it over the neck of a fired, unsized rifle case with the primer removed. Align the two cases between the jaws of your calipers and note the length from rim to rim (See left photo below).
OK, now you have the length for a fired rifle case BEFORE sizing. Next, take a full-length sized rifle case (without primer) and do the same thing, placing the .45 ACP case over the neck of the FL-sized case (Right Photo). The difference between the two numbers is the amount of “bump” or set-back you are applying to the shoulder. Here the difference is .0015″. The amount of bump you need varies with your chamber and your load, but .0015-.002″ is a good initial setting.
If you load pistol ammo you should have a case gauge (aka “gage”) for each cartridge type you reload. Caliber-specific, precision-machined cylindrical gauges perform many important functions. They will instantly reveal if your rounds are too long or have excessive headspace. They will also show if your case is bulged or otherwise too fat to chamber easily. You can use the gauge with sized brass as well as loaded rounds.
Case gauges are a “must-have” for anyone loading handgun ammunition, particularly if you crank out large quantities of pistol ammo with a progressive press. An oversize round can cause a misfeed, jam, or other problem. That can ruin your day if you are in the middle of a shooting match. If you are relying on your handgun for self-defense, the last thing you want is a malfunction of any kind. This Editor personally runs every pistol round through a gauge before it goes into the ammo box.
UltimateReloader.com Video Shows How to Use Pistol Case Gauges:
Our friend Gavin Gear of UltimateReloader.com has prepared an excellent video that explains the benefits of pistol case gauges and shows how to use them. Gavin uses the quality gages produced by L.E. Wilson. These are available for the most popular handgun cartridges, both autoloader cartridges, and rimmed revolver cartridges. Gavin demonstrates gage use with .40 SW and .44 magnum cases.
Gavin states: “Using a case gage is very simple, and I would recommend that you add one of these gages to your reloader’s tool chest for each of the pistol cartridge types you reload. It may just save you a lot of time and hassle. Peace of mind is hard to put a price on!”
Creedmoor Sports is a leading vendor of products for competitive shooters and serious hand-loaders. Along with great gear, Creedmoor Sports provides informative content for its customers. The Creedmoor InfoZone provides Reloading Tips, Gear Reviews, Shooting News, and basic gunsmithing information.
Bill Gravatt, President of Creedmoor Sports, is an expert on reloading processes and gear. He developed many of the popular tools marketed by Sinclair Int’l, and he brings that expertise to Creedmoor Sports. Bill hosts a series of “how-to” videos produced for the Creedmoor InfoZone.
In this video, Bill Gravatt demonstrates several methods to clean your cases. Bill tells us: “Powder residue should be removed before you insert your cases into your reloading dies. There are several ways to clean your cases. Many shooters use a combination of various methods…”
1. Manual Cleaning — You can use 0000 Steel wool for the outside of the case and a Case Neck brush for the inside. A paper towel can remove any remaining residue. This is a handy way to clean if you load at the range.
2. Vibratory Tumbling — This traditional method works well, particularly for pistol brass. Experiment with both Corn Cob and Walnut media. You can get a brighter shine by putting a small amount of liquid brass polish in the media.
3. Wet Tumbling with Stainless Media — This process can get your brass clean inside and out. Do check to ensure no pins are stuck in the flash-holes. Watch for peening of case mouths that can occur over time.
4. Ultrasonic Cleaning — Ultrasonic cleaning works great for small parts as well as brass. The ultrasonic process removes all carbon and traces of lube, which can leave the inside of case necks too dry. To smooth bullet seating, try putting a tablespoon of Ballistol in the cleaning solution.
Cleaning Reloading Dies
Cleaning your reloading dies is something that many hand-loaders neglect. In this 60-second Tech Tip, Bill Gravatt provides some smart advice on cleaning your dies. Bill notes: “After heavy use, case lube and carbon can build up in your reloading dies. It’s important to keep them clean. Also, with new dies, give them a good cleaning before first use, because they ship with a corrosion inhibitor.”
1. Step 1 — Prior to cleaning, disassemble the die and spray it with a good degreaser. Do this with brand new dies too.
2. Step 2 – Take a patch and run it in the die to remove old lube and gunk. Don’t forget the decapping assembly and other internal parts.
3. Step 3 — After cleaning the die, but before reassembly, spray the die with a good corrosion inhibitor, such as Corrosion-X or Starrett M1.
We are often asked “Can I get more velocity by switching primer types?” The answer is “maybe”. The important thing to know is that changing primer types can alter your load’s performance in many ways — velocity average, velocity variance (ES/SD), accuracy, and pressure. Because there are so many variables involved you can’t really predict whether one primer type is going to be better or worse than another. This will depend on your cartridge, your powder, your barrel, and even the mechanics of your firing pin system.
BE SAFE: Be cautious when changing primer types. Glen Zediker recommended decreasing your load ONE FULL GRAIN when changing to a different primer type, one that you haven’t used before.
Interestingly, however, a shooter on another forum did a test with his .308 Win semi-auto. Using Hodgdon Varget powder and Sierra 155gr Palma MatchKing (item 2156) bullets, he found that Wolf Large Rifle primers gave slightly higher velocities than did CCI-BR2s. Interestingly, the amount of extra speed (provided by the Wolfs) increased as charge weight went up, though the middle value had the largest speed variance. The shooter observed: “The Wolf primers seemed to be obviously hotter and they had about the same or possibly better ES average.” See table:
Varget .308 load
45.5 grains
46.0 grains
46.5 grains
CCI BR2 Primers
2751 fps
2761 fps
2783 fps
Wolf LR Primers
2757 fps
2780 fps
2798 fps
Speed Delta
6 fps
19 fps
15 fps
You can’t extrapolate too much from the table above. This describes just one gun, one powder, and one bullet. Your Mileage May Vary (YMMV) as they say. However, this illustration does show that by substituting one component you may see significant changes. Provided it can be repeated in multiple chrono runs, an increase of 19 fps (with the 46.0 grain powder load) is meaningful. An extra 20 fps or so may yield a more optimal accuracy node or “sweet spot” that produces better groups. (Though faster is certainly NOT always better for accuracy — you have to test to find out.)
WARNING: When switching primers, you should exercise caution. More speed may be attractive, but you have to consider that the “speedier” primer choice may also produce more pressure. Therefore, you must carefully monitor pressure signs whenever changing ANY component in a load. In his books, the late Glen Zediker recommended decreasing your load ONE FULL GRAIN when changing to a different primer type, one that you haven’t used before.
What is the most-used piece of equipment on this Editor’s reloading bench? No it’s not my Rock-Chucker press, my priming tool, or even my calipers. The one item in near-constant use is a small, folding magnifying glass. Mine folds into a square case and offers 4X viewing with an 8X bifocal insert. With this handy tool I can inspect case mouths for burrs, check primer pockets, inspect meplats, and look for flaws on bullet jackets. I also use the magnifier to see rifling marks on bullets seated into the rifling, or check my bolt for galling. The number of uses is nearly endless. I keep one magnifier at my reloading bench and another in my range kit.
Folding magnifiers are so handy yet inexpensive that you should own a couple spares (including one in the range box). I bought my magnifier in a book-store, but you can also find them on Amazon.com and FoldingMagnifier.com starting at under $2.00. To see inside cases and dies and view fine details, consider the 5X Meromore Folding Magnifier with a built-in, battery-powered LED light, plus a 11X high-magnification insert.
With a decent small magnifier, you can easily see any minute flaws in your barrel crowns. That’s important because crown damage can cause hard-to-diagnose accuracy issues. We’ve known guys who spend weeks tinkering with loads, when the real problem was a worn-out or damaged crown.
The slide-out iLumen8, shown at left in top photo, offers 3X power with built-in illumination. An “Amazon’s Choice” item, this palm-sized magnifier is $20.75 on Amazon.
The LEE Hand Press Kit comes complete with: Breech Lock Hand Press, Ram Primer fittings, Powder Funnel, Case Lube, and one Breech Lock Quick Change Bushing.
The LEE Breech Lock Hand Press is a useful addition to any reloader’s tool collection. This Hand Press allows you to resize brass or seat bullets while at the range or away from your loading room. No it won’t replace your Co-Ax or Rockchucker, but this is a handy piece of gear. And it’s very affordable. Midsouth sells the LEE Breech Lock Hand Press Kit, with everything shown above, for $58.89. Or you can buy just the Hand Press by itself for $42.22 at Battlehawk Armory. That’s a great deal.
LEE Hand Press Quick-Change Die Functionality
This LEE Press includes the Breech Lock Quick-Change Die Feature, so you can instantly change dies with just a twist of the wrist. One Breech Lock Quick Change Bushing is included. The Breech Lock Hand Press is light and compact. It tips the scale at a mere 1 3/4 pounds and overall product length is 11.5″. Note: the maximum overall length cartridge the LEE Hand Press can accept is 3.650 inches.
While we don’t recommend the Hand Press for major full-length sizing chores, it does a good job seating bullets. This makes it a handy “field expedient” tool when doing load development at the range. For example you can experiment with seating depth by loading a batch of cartridges at home with the bullets deliberately seated long. Then, at the range, you can re-seat the bullets progressively shorter (in a series of rounds), until you find the optimal Cartridge OAL.
LEE’s Hand Press is also great for decapping operations. One verified buyer reports: “I mostly use it for decapping while watching TV, with a universal decapper die. Of course it is also useful for other operations. I wouldn’t love sizing large rifle cases with it, but if you’ve prepped your cases on a bigger press you could comfortably take it to the range for load development, one round at a time, working right there at the line instead of making a bunch of loads at home that you’ll have to disassemble later. It’s smaller and more lightweight than it looks[.]”
Using LEE Hand Press to Load at the Range
To learn more about using the LEE Hand Press in the field with a compact scale, read Reloading Without a Bench on GunsAmerica.com. Author Christopher Mace explains: “whether you want a better way to do load development, the capability to reload out of your ‘bugout bag’, or you’re short of space but wanting to get into reloading, the LEE Loaders and the LEE Hand Press offer you some options.”
If you are considering purchasing a progressive reloading press — you’ve struck gold. Today’s Saturday at the Movies article showcases the most comprehensive video ever created about progressive presses. Hosted by Gavin Gear of UltimateReloader.com, this remarkable 79-minute video covers a dozen presses from six leading manufacturers: Dillon, Frankford Arsenal, Hornady, Lee, Mark 7 (Lyman), and RCBS.
Set aside plenty of time, because there is a wealth of information — the “mother lode” of progressive press coverage. Along with the big comparison video, Gavin has prepared a detailed, 17-page online article which covers all of the presses in the shootout. This 17-page article also includes many product-specific videos. We link to five of these videos below.
12 Progressive Presses are compared in this comprehensive video:
Here it is! Ultimate Reloader’s long-awaited Progressive Press Shootout. This online article and accompanying video represent the most exhaustive and most in-depth look at progressive press reloading equipment. The Ultimate Reloader Progressive Shootout covers a dozen different progressives from Dillon, Frankford Arsenal, Hornady, Mark 7 (Lyman), and RCBS. Reviewer Gavin Gear demonstrates how each press works and provides data on costs, output rates, capabilities, and accessories so that potential buyers can make informed purchasing decisions.
MORE INFO — Progressive Press Shootout Online Article
To accompany the remarkable 79-minute Progressive Press Shootout Video, Gavin Gear has posted a ton of information on his UltimateReloader.com website. A lengthy online article provides detailed information on the particular presses, press mounts, and lighting, as well as general details such as cost of ownership. We provide links to particular topics below. This is a GREAT RESOURCE — it’s like getting an entire chapter of a technical book all for FREE!
Below we provide links to each product-specific online page, along with the corresponding time-link to the related segment of the 79-minute Progressive Shootout video.
NOTE: Along with the presses featured in these six videos, the Ultimate Reloader Progressive Press Comparison video covers six other presses (12 total), in a comprehensive 1 hour 19 minute video linked at the top of this article.
by Sierra Bullets Ballistic Technician Paul Box
One thing that plays a major role in building an accuracy load is neck tension [one of the factors that controls the “grip” on a bullet]. I think a lot of reloaders pretty much take this for granted and don’t give that enough thought.
So, how much neck tension is enough?
Through the years and shooting both a wide variety of calibers and burn rates of powder, I’ve had the best accuracy overall with .002″ of neck tension. Naturally you will run into a rifle now and then that will do its best with something different like .001″ or even .003″, but .002″ has worked very well for me. So how do we control the neck tension? Let’s take a look at that.
First of all, if you’re running a standard sizing die with an expander ball, just pull your decapping rod assembly out of your die and measure the expander ball. What I prefer [for starters] is to have an expander ball that is .003″ smaller than bullet diameter. So for example in a .224 caliber, run an expander ball of .221″. If you want to take the expander ball down in diameter, just chuck up your decapping rod assembly in a drill and turn it down with some emery cloth. When you have the diameter you need, polish it with three ought or four ought steel wool. This will give it a mirror finish and less drag coming through your case neck after sizing.
Tips for Dies With Interchangeable Neck Bushings
If you’re using a bushing die, I measure across the neck of eight or ten loaded rounds, then take an average on these and go .003″ under that measurement. There are other methods to determine bushing size, but this system has worked well for me.
Proper Annealing Can Deliver More Uniform Neck Tension
Another thing I want to mention is annealing. When brass is the correct softness, it will take a “set” coming out of the sizing die far better than brass that has become too hard. When brass has been work hardened to a point, it will be more springy when it comes out of a sizing die and neck tension will vary. Have you ever noticed how some bullets seated harder than others? That is why.
Paying closer attention to neck tension will give you both better accuracy and more consistent groups.