Each Wednesday, the U.S. Army Marksmanship Unit publishes a reloading “how-to” article on the USAMU Facebook page. This past week’s “Handloading Hump Day” article, the latest in a 7-part series, relates to chronograph testing and statistical samples. We highly recommend you read this article, which offers some important tips that can benefit any hand-loader. Visit the USAMU Facebook page next Wednesday for the next installment.
Chronograph Testing — Set-Up, Sample Sizes, and Velocity Factors
Initial Chronograph Setup
A chronograph is an instrument designed to measure bullet velocity. Typically, the bullet casts a shadow as it passes over two electronic sensors placed a given distance apart. The first screen is the “start” screen, and it triggers an internal, high-speed counter. As the bullet passes the second, or “stop” screen, the counter is stopped. Then, appropriate math of time vs. distance traveled reveals the bullet’s velocity. Most home chronographs use either 2- or 4-foot spacing between sensors. Longer spacing can add some accuracy to the system, but with high-quality chronographs, 4-foot spacing is certainly adequate.
Laboratory chronographs usually have six feet or more between sensors. Depending upon the make and model of ones chronograph, it should come with instructions on how far the “start” screen should be placed from one’s muzzle. Other details include adequate light (indoors or outdoors), light diffusers over the sensors as needed, and protecting the start screen from blast and debris such as shotgun wads, etc. When assembling a sky-screen system, the spacing between sensors must be extremely accurate to allow correct velocity readings.
Statistics: Group Sizes, Distances and Sample Sizes
How many groups should we fire, and how many shots per group? These questions are matters of judgment, to a degree. First, to best assess how ones ammunition will perform in competition, it should be test-fired at the actual distance for which it will be used. [That means] 600-yard or 1000-yard ammo should be tested at 600 and 1000 yards, respectively, if possible. It is possible to work up very accurate ammunition at 100 or 200 yards that does not perform well as ranges increase. Sometimes, a change in powder type can correct this and produce a load that really shines at longer range.
The number of shots fired per group should be realistic for the course of fire. That is, if one will be firing 10-shot strings in competition then final accuracy testing, at least, should involve 10-shot strings. These will reflect the rifles’ true capability. Knowing this will help the shooter better decide in competition whether a shot requires a sight adjustment, or if it merely struck within the normal accuracy radius of his rifle.
How many groups are needed for a valid test? Here, much depends on the precision with which one can gather the accuracy data. If shooting from a machine rest in good weather conditions, two or three 10-shot groups at full distance may be very adequate. If it’s windy, the rifle or ammunition are marginal, or the shooter is not confident in his ability to consistently fire every shot accurately, then a few more groups may give a better picture of the rifle’s true average.
Who hasn’t dreamed of having a professional-quality, permanent shooting bench on their own property? Well here’s an article that can help you make that dream come true. This “how-to” feature from the archives of RifleShooter Magazine shows how to build a quality concrete shooting bench step-by-step.
All aspects of the construction process are illustrated and explained. The author, Keith Wood explains: “Construction happened in three phases — first creating the slab foundation, then the support pillars (legs), and finally the table.”
Click image below to load article with slide show.
Each step in the process is illustrated with a large photo and descriptive paragraph. Starting with framing the foundation (Step 1), the article illustrates and explains the 15 Steps that produce the finished, all-concrete bench (see top photo).
Flat-bottomed stocks are great for benchrest shooting, but their geometry is not ideal for mounting conventional Harris bipods, which were originally designed for stocks with a curved underbelly. Long-time Forum member Mark S. wanted to know if there is a way to make a stud-mounted bipod more secure on a flat-bottomed stock: “I have started shooting some steel matches that require shooting from bipods. My best gun for the job is a 6BRX in a MBR benchrest stock. I have installed a stud, but the bipod is still wanting to turn sometimes. What do you use?”
Here’s a solution for Mark and others using Harris bipods on flat-bottomed stocks with studs. Get the Harris-made #9 (HB9) adapter. Costing just $22.12 (at Midsouth), the HB9 adapter provides an extended contact surface with pads, so the bipod will fit securely on your flat fore-end.The HB9 adapter also has a center cut-out for the swivel stud so the bipod adapter aligns properly on the underside of your stock:
Planning a rimfire build in the future, or want to get the exact specifications for your 17 Mach 2, 17 HMR, 17 WSM, .22 LR, or .22 WMR rifle? Then we’ve got you covered. CLICK HERE to download an official SAAMI document with specs for all popular rimfire ammunition. This FREE 89-page PDF includes complete Cartridge and Chamber drawings and headspace specifications. The document even includes info on pressure terminology, headspace gauges, and test barrels.
SAAMI, the Sporting Arms and Ammunition Manufacturers’ Institute, is an association of the nation’s leading manufacturers of firearms, ammunition and components. Founded in 1926, SAAMI promulgates industry standards for firearms ammunition safety, interchangeability, reliability and quality.
SAAMI offers diagrams for ALL popular, modern rimfire cartridges:
Here’s the Official SAAMI Drawing for the 17 HMR Cartridge:
SAAMI Website Also Offers Centerfire Cartridge and Shotshell Diagrams
From the SAAMI website you can also access hundreds of FREE official cartridge diagrams and chamber drawings for all centerfire ammo types and Shotshells. Click on the Cartridge and Chamber Drawings link, then go to the SAAMI Standards Page. On the Standards Page, choose your ammo type: Rimfire (Z299.1), Shotshell (Z299.2), Pistol and Revolver (Z299.3), Rifle (Z299.4). This will then load large PDF files which include multiple cartridge diagrams.
Let’s face it, AR-platform rifles run dirty, at least compared to typical bolt-action rifles. The AR15 works by piping gas from the barrel back into the bolt carrier, causing the bolt to unlock and the carrier to move the bolt backward. The “exhaust gas” from the barrel contains soot and carbon. The carbon will form hard deposits on the bolt. In addition, the carbon can combine with lube on the bolt carrier to make a nasty, paste-like sludge. This can be particularly problematic when the black paste pollutes the ejector and extractor recess.
This Editor has inspected dozens of ARs over the years. Other than mag-related malfunctions, the most common cause of AR cycling problems I found was oily gunk in the extractor and ejector areas. Many AR owners overlook these critical areas. Look at an AR that hasn’t been cleaned properly and you’ll probably find black gunk (and small brass shavings) in the ejector and extractor recesses.
If you want to keep your black rifle running smoothly and reliably, you must clean it regularly and follow the correct maintenance procedures. Here are three videos that explain how to properly disassemble and clean AR-platform rifles.
Take-Down and Full Cleaning of AR15 by Jerry Miculek
Here ace shooter Jerry Miculek takes down and cleans an AR-platform rifle belonging to his daughter Lena. This is a good video because Lena’s rifle was “run hard and packed up dirty” so you can see where carbon and grease build up. This 35-minute video is very thorough. Jerry is one of the nation’s top action carbine shooters, so listen carefully to his advice on cleaning and lubrication.
How to Clean Your AR-Platform Rifle
This is a good basic video that shows the take-down and cleaning procedure for a typical AR15. It uses some fast-motion sequences to speed up the story. Check out this video if you don’t have the time to sit through the Miculek video above.
Cleaning and Lubricating AR15 Bolt Carrier Group
This video offers very specific advice on the bolt carrier group, which receives the dirty gas directly from the barrel. Be sure to check the extractor and ejector recesses. That’s where old lube, brass shavings, and carbon accumulate. Follow the directions in this video for lubrication, and don’t over-lubricate the bolt carrier — that will only capture more carbon.
Shooters contemplating purchase of a .338 LM rifle often ask: “What length barrel should I get?” Rifleshooter.com recently performed a test that provides interesting answers…
Our friends at RifleShooter.com like to slice and dice — barrels that is. They have done barrel length cut-down tests for popular calibers like the .223 Rem, 6.5 Creedmoor, and .308 Winchester. But now they’ve tackled something way bigger — the .338 Lapua Magnum, a true “Big Boomer”. Starting with a beefy 30″-long Pac-Nor Barrel, RifleShooter.com chopped the tube down in one-inch increments all the way down to 17 inches (that’s 14 different lengths). At each new (shorter) barrel length, velocity was measured with a MagnetoSpeed chronograph using two different loads, 250gr SMKs with H4831sc and 300gr SMKs with Retumbo. Four shots were fired at each length with each load, a total of 112 rounds.
Load #1: 250gr Sierra MK, Lapua brass, CCI #250 primer, H4831SC, OAL 3.720″.
Load #2: 300gr Sierra MK, Lapua brass, Win WLRM primer, Retumbo, OAL 3.720″.
The .338 Lapua Magnum is a jumbo-sized cartridge, that’s for sure…
Donor Barrel Sacrificed for Science
Rifleshooter.com’s Editor explains: “Brandon from Precision Addiction offered to send us his .338 barrel for our .338 Lapua Mag test. I took him up on his offer and he sent me his used Pac-Nor chrome-moly barrel with about 600 rounds though it. This thing was a beast! A heavy 1.350″ shank that ran straight for 6″, until tapering to 1″ at 30″ in length.”
Results Summary
.338 Lapua Magnum with 250gr Sierra MatchKings
After shortening the barrel from 30″ to 17″, total velocity reduction for the 250-grainers was 395 FPS, an average loss of 30.4 FPS per 1″ cut. The amount of velocity loss per inch rose as the barrel got shorter, with the biggest speed reduction, a loss of 55 FPS, coming with the cut from 18″ to 17″.
Start Velocity: 2942 FPS | End Velocity: 2547 FPS | Average Loss Per Inch: 30.4 FPS
.338 Lapua Magnum with 300gr Sierra MatchKings
Shooting the 300-grainers, total velocity reduction was 341 fps, an average of 26.2 FPS loss per 1″ cut (30″ down to 17″). However, the speed actually increased with the first cut from 30 inches to 29 inches. The tester noted: “The 300 SMK load showed a slight increase from 30″ to 29″. I’ve recorded this in other tests and it seems to be more common with a heavier load. I suspect it is primarily due to the small sample sizes being used along with the relative proximity of muzzle velocities in adjacent lengths.”
Start Velocity: 2833 FPS | End Velocity: 2492 FPS | Average Loss Per Inch: 26.2 FPS*
*Velocity rose with first cut. Velocities ranged from 2,871 FPS (29″) to 2,492 FPS (17″) for a total velocity loss of 341 FPS.
RifleShooter.com crunched the velocity numbers in some interesting ways. For example they analyzed rate of velocity loss, concluding that: “after the initial rate change, the rate of the change in velocity is fairly consistent.” (View Rate of Change Graph)
How Velocity Loss Alters Long-Range Ballistics
The testers wanted to determine how the velocity reductions “affect our ability to hit targets downrange”. So, Rifleshooter.com plotted changes in elevation and wind drift at all barrel lengths. This revealed something interesting — drift increased significantly below 26″ barrel length: “Above 26″ things look pretty good, below 22″ they change quickly.”
We highly recommend you read the whole story. Rifleshooter.com put in serious time and effort, resulting in solid, thought-provoking results. The data is presented in multiple tables and graphs, revealing inch-by-inch velocities, change “deltas”, and SDs at each length.
Here’s a little known fact that may startle most readers, even experienced gunsmiths: your barrel wears out in a matter of seconds. The useful life of a typical match barrel, in terms of actual bullet-in-barrel time, is only a few seconds. How can that be, you ask? Well you need to look at the actual time that bullets spend traveling through the bore during the barrel’s useful life. (Hint: it’s not very long).
Bullet-Time-in-Barrel Calculations
If a bullet flies at 3000 fps, it will pass through a 24″ (two-foot) barrel in 1/1500th of a second. If you have a useful barrel life of 3000 rounds, that would translate to just two seconds of actual bullet-in-barrel operating time.
Ah, but it’s not that simple. Your bullet starts at zero velocity and then accelerates as it passes through the bore, so the projectile’s average velocity is not the same as the 3000 fps muzzle velocity. So how long does a centerfire bullet (with 3000 fps MV) typically stay in the bore? The answer is about .002 seconds. This number was calculated by Varmint Al, who is a really smart engineer dude who worked at the Lawrence Livermore Laboratory, a government think tank that develops neutron bombs, fusion reactors and other simple stuff.
On his Barrel Tuner page, Varmint Al figured out that the amount of time a bullet spends in a barrel during firing is under .002 seconds. Al writes: “The approximate time that it takes a 3300 fps muzzle velocity bullet to exit the barrel, assuming a constant acceleration, is 0.0011 seconds. Actual exit times would be longer since the bullet is not under constant acceleration.”
We’ll use the .002 number for our calculations here, knowing that the exact number depends on barrel length and muzzle velocity. But .002 is a good average that errs, if anything, on the side of more barrel operating life rather than less.
So, if a bullet spends .002 seconds in the barrel during each shot, and you get 3000 rounds of accurate barrel life, how much actual firing time does the barrel deliver before it loses accuracy? That’s simple math: 3000 x .002 seconds = 6 seconds.
Gone in Six Seconds. Want to Cry Now?
Six seconds. That’s how long your barrel actually functions (in terms of bullet-in-barrel shot time) before it “goes south”. Yes, we know some barrels last longer than 3000 rounds. On the other hand, plenty of .243 Win and 6.5-284 barrels lose accuracy in 1500 rounds or less. If your barrel loses accuracy at the 1500-round mark, then it only worked for three seconds! Of course, if you are shooting a “long-lived” .308 Win that goes 5000 rounds before losing accuracy, then you get a whopping TEN seconds of barrel life. Anyway you look at it, a rifle barrel has very little longevity, when you consider actual firing time.
People already lament the high cost of replacing barrels. Now that you know how short-lived barrels really are, you can complain even louder. Of course our analysis does give you even more of an excuse to buy a nice new Bartlein, Krieger, Shilen etc. barrel for that fine rifle of yours.
To err is human… Sooner or later you’ll probably get a case stuck in a die. This “fix-it” article, which originally appeared in the Western Powders Blog, explains the procedure for removing a firmly stuck cartridge case using an RCBS kit. This isn’t rocket science, but you do want to follow the directions carefully, step-by-step. Visit the Western Powders Blog for other helpful Tech Tips.
Curing the Stuck Case Blues
Sticking a case in the sizer die is a rite of passage for the beginning handloader. If you haven’t done it yet, that’s great, but it probably will eventually happen. When it does, fixing the problem requires a bit of ingenuity or a nice little kit like the one we got from RCBS.
The first step is to clear the de-capping pin from the flash hole. Just unscrew the de-capping assembly to move it as far as possible from the primer pocket and flash hole (photo at right). Don’t try to pull it all the way out. It won’t come. Just unscrew it and open as much space as possible inside the case.
Place the die upside down in the padded jaws of a vise and clamp it firmly into place. Using the supplied #7 bit, drill through the primer pocket. Be careful not to go too deeply inside the cartridge once the hole has opened up. It is important to be aware that the de-capping pin and expander ball are still in there and can be damaged by the bit.
Drill and Tap the Stuck Case
Once the cartridge head has been drilled, a ¼ – 20 is tap is used to cut threads into the pocket. Brass is relatively soft compared to a hardened tap, so no lube is needed for the tapping process. RCBS says that a drill can be used for this step, but it seems like a bit of overkill in a project of this nature. A wrench (photo above right) makes short work of the project.
RCBS supplies a part they call the “Stuck Case Remover Body” for the next step. If you are a do-it-yourselfer and have the bit and tap, this piece is easily replicated by a length of electrical conduit of the proper diameter and some washers. In either case, this tool provides a standoff for the screw that will do the actual pulling.
With an Allen Wrench, Finish the Job
Run the screw through the standoff and into the tapped case head. With a wrench, tighten the screw which hopefully pulls the case free. Once the case is free, clamp the case in a vice and pull it free of the de-capping pin. There is tension here because the sizing ball is oversized to the neck dimension as part of the sizing process. It doesn’t take much force, but be aware there is still this last little hurdle to clear before you get back to loading. Don’t feel bad, everyone does this. Just use more lube next time!
Article find by EdLongrange. We welcome reader submissions.
Berger Twist-Rate Stability Calculator
On the updated Berger Bullets website you’ll find a handy Twist-Rate Stability Calculator that predicts your gyroscopic stability factor (SG) based on mulitiple variables: velocity, bullet length, bullet weight, barrel twist rate, ambient temperature, and altitude. This very cool tool tells you if your chosen bullet will really stabilize in your barrel.
LIVE DEMO BELOW — Just enter values in the data boxes and click “Calculate SG”.
How to Use Berger’s Twist Rate Calculator
Using the Twist Rate Calculater is simple. Just enter the bullet DIAMETER (e.g. .264), bullet WEIGHT (in grains), and bullet overall LENGTH (in inches). On its website, Berger conveniently provides this info for all its bullet types. For other brands, we suggest you weigh three examples of your chosen bullet, and also measure the length on three samples. Then use the average weight and length of the three. To calculate bullet stability, simply enter your bullet data (along with observed Muzzle Velocity, outside Temperature, and Altitude) and click “Calculate SG”. Try different twist rate numbers (and recalculate) until you get an SG value of 1.4 (or higher).
Gyroscopic Stability (SG) and Twist Rate
Berger’s Twist Rate Calculator provides a predicted stability value called “SG” (for “Gyroscopic Stability”). This indicates the Gyroscopic Stability applied to the bullet by spin. This number is derived from the basic equation: SG = (rigidity of the spinning mass)/(overturning aerodynamic torque).
At the Berger SW Nationals this week in Phoenix, the nation’s top long-range shooters will try to put all their shots in the 10-Ring at 800, 900, and 1000 yards. A good foundation in ballistics is vital if you want to succeed in the long-range game.
How much do you know about BCs, Bullet Shapes, Trajectories, Wind Drift, and other things in the realm of External Ballistics? You can test your knowledge of basic Ballistics principles with this interactive quiz. The questions and answers were provided by Ballistics Guru Bryan Litz of Applied Ballistics LLC. Bryan is the author of Applied Ballistics for Long-Range Shooting and other popular resources in print, DVD, and eBook format. Have fun with our Quiz.
The Quiz contains ten (10) questions. When you complete all ten questions, you can see your results, along with the correct answers.
Flat-bottomed stocks are great for benchrest shooting, but their geometry is not ideal for mounting conventional Harris bipods, which were originally designed for stocks with a curved underbelly. Long-time Forum member Mark S. wanted to know if there is a way to make a stud-mounted bipod more secure on a flat-bottomed stock: “I have started shooting some steel matches that require shooting from bipods. My best gun for the job is a 6BRX in a MBR benchrest stock. I have installed a stud, but the bipod is still wanting to turn sometimes. What do you use?”
Gone in Six Seconds. Want to Cry Now?
Sticking a case in the sizer die is a rite of passage for the beginning handloader. If you haven’t done it yet, that’s great, but it probably will eventually happen. When it does, fixing the problem requires a bit of ingenuity or a nice little kit like the one we got from RCBS.
