Here are two different bullet types, seated to the same CBTO length, but different COAL. Note the shiny scratches on the bullets made by the comparator tool which indicates a point on the bullet ogive near where the ogive will engage the rifling.
Effects Of Cartridge Over All Length (COAL) And Cartridge Base To Ogive (CBTO) – Part 2 by Bryan Litz forBerger Bullets. Part One of this series focused on the importance of COAL in terms of SAAMI standards, magazine lengths, seating depths, and pressure levels. Another measure of length for loaded ammunition is highly important to precision, namely Cartridge Base to Bullet Ogive Length (CBTO).
Figure 2. Chamber throat geometry showing the bullet jump to the rifling or lands.
Look at Figure 2. Suppose the bullet was seated out of the case to the point where the base of the bullet’s nose (ogive) just contacted the beginning of the riflings (the lands) when the bolt was closed. This bullet seating configuration is referred to as touching the lands, or touching the riflings and is a very important measurement to understand for precision hand-loading. Due to the complex dynamics of internal ballistics which happen in the blink of an eye, the distance a bullet moves out of the case before it engages the riflings is highly critical to precision potential. Therefore, in order to systematically optimize the precision of his handloads, it’s critically important that the precision hand-loader understands how to alter bullet seating depth in relation to the barrel rifling. Part of the required knowledge is understanding how to accurately and repeatably measure the Cartridge Base To Ogive (CBTO) dimension. This is explained in the FULL ARTICLE.
Bryan Litz offers an extended discussion on how to measure CBTO using different tools and methods, including the Hornady OAL gauge. You can read this discussion in the full article found on the Berger Bullets website. CLICK HERE to Read Full Article.
Why Not Use CBTO as a SAAMI Standard?
If CBTO is so important to rifle accuracy, you might ask, “Why is it not listed as the SAAMI spec standard in addition to COAL?” There is one primary reason why it is not listed in the standard. This is the lack of uniformity in bullet nose shapes and measuring devices used to determine CBTO.
Benefits of Having a Uniform CBTO
There is another aspect to knowing your CBTO when checking your COAL as it pertains to performance. With good bullets, tooling, and carefully-prepared cases you can easily achieve a CBTO that varies less than +/- .001″ but your COAL can vary as much as .025″ extreme spread (or more with other brands). This is not necessarily bad and it is much better than the other way around. If you have a CBTO dimension that varies but your COAL dimension is tight (within +/- .002″) then it is most likely that your bullet is bottoming out inside the seater cone on the bullet tip. This is very bad and is to be avoided. It is normal for bullets to have precisely the same nose shape and it is also normal for these same bullets to have nose lengths that can vary as much as .025″.
Summary of Cartridge Base To Ogive (CBTO) Discussion
Here are four important considerations regarding bullet seating depth as it relates to CBTO:
1. CBTO is a critical measurement to understand for handloaders because it’s directly related to precision potential, and you control it by simply setting bullet seating depth.
2. Tools and methods for measuring CBTO vary. Most of the measurement techniques have pitfalls (which may give rise to inconsistent results) that you should understand before starting out.
3. A CBTO that produces the best precision in your rifle may not produce the best precision in someone else’s rifle. Even if you have the same rifle, same bullets, same model of comparator gauges, etc. It’s possible that the gauges are not actually the same, and measurements from one don’t translate to the same dimension for another.
4. Once you find the CBTO that produces the best precision in your rifle, it’s important to allow minimal variation in that dimension when producing quality handloads. This is achieved by using quality bullets, tooling, and properly preparing case mouths and necks for consistent seating.
To get the best accuracy out of any benchrest rifle, you need to find the optimal position of front rest and rear bag. The important point to remember is that each rig is different. One gun may perform best with the front rest right at the tip of the forearm (Position ‘D’ in photo), while another gun will work best with the rest positioned much further back. This Editor’s own 6mmBR rifle has a laminated stock that is pretty flexy in the front. It shoots best with the front rest’s sandbag located a good 6″ back from the forearm tip (position ‘A’).
Here’s some benchrest advice that can help you reduce vertical and shoot tighter groups… without spending another penny. Many benchrest shooters spend a fortune on equipment and devote countless hours to meticulous handloading, but they never experiment with their rifle’s position/balance on the bags. This article explains why you should test your rifle in various positions. What you learn may surprise you (and improve your scores).
Next time you go to the range, experiment with the position of your rifle on the front rest, and try a couple different positions for the rear bag. You may find that the rifle handles much better after you’ve made a small change in the placement of your gun on the bags. Recoil can be tamed a bit, and tracking can improve significantly, if you optimize the front rest and rear bag positioning.
This competitor has the front rest positioned fairly far forward but not all the way out. Note the stop on the front rest — this limits forward stock travel.
A small change in the position of the forearm on the front rest, or in the placement of the rear bag, can make a big difference in how your gun performs.
Balance Your Gun BEFORE You Spend Hours Tuning Loads
In the pursuit of ultimate accuracy, shooters may spend countless hours on brass prep, bullet selection, and load tuning. Yet the same shooters may pay little attention to how their gun is set-up on their front rest and rear bag. When you have acquired a new rifle, you should do some basic experimentation to find the optimal position for the forearm on the front rest, and the best position for the rear bag. Small changes can make a big difference.
Joel Kendrick, past IBS 600-yard Shooter of the Year, has observed that by adjusting forearm position on the front rest, he can tune out vertical. He has one carbon-fiber-reinforced stock that is extremely rigid. When it was placed with the front rest right under the very tip of the forearm, the gun tended to hop, creating vertical. By sliding the whole gun forward (with more forearm overhang ahead of the front sandbag), he was able to get the whole rig to settle down. That resulted in less vertical dispersion, and the gun tracked much better.
Fore/aft stock position is important even with very wide fore-ends.
Likewise, the placement of the rear bag is very important. Many shooters, by default, will simply place the rear bag the same distance from the front rest with all their guns. In fact, different stocks and different calibers will NOT behave the same. By moving the rear bag forward and aft, you can adjust the rifle’s overall balance and this can improve the tracking significantly. One of our shooters had a Savage 6BR F-Class rifle. By default he had his rear bag set almost all the way at the end of the buttstock. When he slid the rear bag a couple inches forward the gun tracked much better. He immediately noticed that the gun returned to point of aim better (crosshairs would stay on target from shot to shot), AND the gun torqued (twisted) less. The difference was quite noticeable.
A small change in the position of the forearm on the front rest, or in the placement of the rear bag, can make a big difference in how your gun performs. You should experiment with the forearm placement, trying different positions on the front rest. Likewise, you can move the rear bag back and forth a few inches. Once you establish the optimal positions of front rest and rear bag, you should find that your gun tracks better and returns to battery more reliably. You may then discover that the gun shoots smaller groups, with less vertical dispersion. And all these benefits are possible without purchasing any expensive new gear.
Are you a do-it-yourself kind of guy with a creative eye? Then you’ll love the FREE Target Generator from the folks at ShooterShed.com. This FREE, interactive webpage allows you to design a variety of fun targets, including grids, benchrest-type Score/Group Shooting targets, sight-in targets, and even playing card targets. Choose the paper size and orientation (vertical or horizontal), then select the number of target elements on the page. For example, you could have four (4) bulls or a dozen playing cards. Then click the Style TAB to choose your target style. Use the OPTIONS TAB to overlay a grid on the target, choose squares or diamonds, or include load information blocks. For bullseye targets, you can control the number, color, and spacing (diameter) of the rings. LINK to TARGET GENERATOR.
QWIK TIP: For Super-Quick Target Creation, click the “Target Style” tab, then select a design from the list on the left. Then go to OPTIONS.
Click the TARGET STYLE TAB to select one of many target styles, including NBRSA Benchrest targets and 20+ types of NRA bullseye targets, scaled to distance:
Benchrest
Bullseye
Shapes
NRA High Power
NRA Pistol
NRA Rimfire
Images
Playing Cards
Load Test Blocks
Creating new targets is fast and easy. No computer graphics skills required! We created this green diamonds grid target in just five minutes using the ShooterShed Target Generator:
And here is a handy target with short-range range Benchrest competition-style box/circle aiming points, along with fields for entry of gun/load data:
The program provides a preview of each target you generate. There are controls to choose border and fill colors. If you like a particular design, save the file, and then print as many targets as you want. Check it out, this program is fun and handy to use. Here are four (4) targets your Editor created just for this article. With a bit of practice, you can be generating your own custom targets in minutes.
About the Creator of the Target Generator
The Target Generator program was created by Rod Brown of Sheridan, Wyoming. Rod tells us: “I build custom rifles… I’ve got a 100-yard range out my back door. I shoot short- and long-range benchrest competitively around the country. I’m a full-time software development consultant and an FFL holder. When I’m not developing custom software for my clients, I’m usually fiddling in the shop, building a custom benchrest rifle, traveling to a match, chambering a barrel, or reloading some ammunition.”
Story tip from Boyd Allen. We welcome reader submissions.
Will you be traveling out of state this spring? Or do you have questions about your own state’s firearms laws? Then you’ll want to have quick access to summaries of gun laws in America’s 50 states. Here’s a very valuable online resource you may want to bookmark.
The NRA-ILA (Institute for Legislative Action) has a great feature on the NRA-ILA’s homepage. There is a pull-down menu that includes all 50 states plus the District of Columbia, New York City, and Guam. This quickly provides a host of legal information for each region you select. From the NRA home page, NRA-ILA.org, you can instantly access a host of information for all 50 states. NOTE: Some of this information is not completely current, so you may want to check with more detailed, up-to-date reference, such as the Legal Heat $10 50 State Guide to Firearms Laws and Regulations (2023 Edition). Most of the information in the book is also available in the FREE Legal Heat Firearms Law APP for Android (Google) smartphones and Android mobile devices. Here are screenshots from the Legal Heat Android App.
Shown below is part of the NRA-ILA’s coverage for Texas. There is a summary of the most important Texas gun laws. Below that is a map showing the states that recognize Texas carry permits.
Today, June 20, 2024 is Summer Solstice, the longest day of the year. And right now the Northeast USA is undergoing a major heatwave. That means “peak heat” summer conditions have arrived already. It’s vitally important to keep your ammo at “normal” temps during the hot summer months. Even if you use “temp-insensitive” powders, studies suggest that pressures can still rise dramatically when the entire cartridge gets hot, possibly because of primer heating.
It’s smart to keep your loaded ammo in an insulated storage unit, possibly with a Blue Ice Cool Pak if you expect it to get quite hot. Don’t leave your ammo in the car or truck — temps can exceed 140° in a vehicle parked in the sun.
The way ambient temperatures, barrel heating, and powder warming can affect cartridge pressures (and hence velocities) was covered in a study Pressure Factors: How Temperature, Powder, and Primer Affect Pressure by Denton Bramwell. In that article, the author used a pressure trace instrument to analyze how temperature affects ammo performance. Bramwell’s tests yielded some fascinating results.
For example, barrel temperature was a key factor: “Both barrel temperature and powder temperature are important variables, and they are not the same variable. If you fail to take barrel temperature into account while doing pressure testing, your test results will be very significantly affected. The effect of barrel temperature is around 204 PSI per F° for the Varget load. If you’re not controlling barrel temperature, you about as well might not bother controlling powder temperature, either. In the cases investigated, barrel temperature is a much stronger variable than powder temperature.”
This Editor had the personal experience of 6mmBR hand-loaded ammo that was allowed to sit in the hot sun for 45 minutes while steel targets were reset. The brass became quite warm to the touch, meaning the casings were well over 120° on the outside. When I then shot this ammo, the bullets impacted well high at 600 yards (compared to earlier in the day). Using a Magnetospeed, I then chron-tested the sun-heated ammo. The hot ammo’s velocity FPS had increased very significantly — all because I had left the ammo out in the hot sun uncovered for 3/4 of an hour.
LESSON: Keep your ammo cool! Keep loaded ammo in the shade, preferably under cover or in an insulated container. You can use a SEALED cool pack inside the container, but we do NOT recommend H20 ice packs. And don’t have the container do double duty for food and beverages.
Powder Heat Sensitivity Comparison Test
Our friend Cal Zant of the Precision Rifle Blog has published a fascinating comparison test of four powders: Hodgdon H4350, Hodgdon Varget, IMR 4451, and IMR 4166. The first two are Hodgdon Extreme powders, while the latter two are part of IMR’s Enduron line of propellants.
The testers measured the velocity of the powders over a wide temperature range, from 25° F to 140° F. Hodgdon H4350 proved to be the most temp stable of the four powders tested. [NOTE: New Alliant Reloder TS 15.5 has also proved very temp stable in AccurateShooter’s range tests.]
Even wonder how a pump shotgun works? Then watch this fascinating video from MidwayUSA. The operation of a pump-action shotgun is illustrated with a special cut-away version of a Winchester Model 12. The shotgun has been modified to reveal the inner workings. This cut-away Model 12 still loads and ejects dummy shells, but you can see how the lugs, slides, locks, ramps, springs and other internal parts work. You’ll be amazed how complicated this old pump-gun is. (The Model 12, Winchester’s first hammerless shotgun, is one of the most popular scatterguns ever made. Over 2,000,000 were sold.)
Skip Ahead to 3:00 to See Cut-Away in Action
To see how the Model 12 works, you can skip forward to the 3:00 minute mark in the video. The first part of the video shows how the Model 12 was “sliced and diced” to expose the inner workings. Larry Potterfield of MidwayUSA explains that “the factories often used cut-aways as sales tools to show how a specific model operated”. In addition the U.S. Military used cut-aways for training purposes.
Here is the cut-away completed. Even the pump grip has been sliced to reveal the inner workings.
A round has been picked up from the feed tube, and then is lifted into the chamber.
Over the past 12 months, this article was one of the TOP 20 most-read Daily Bulletin features. We’re reprising it today for those who may have missed it the first time. The above diagram comes from a TiborasurasRex YouTube Video comparing G1 and G7 BC models. CLICK HERE to watch the video.
The better, up-to-date ballistics programs let you select either G1 or G7 Ballistic Coefficient (BC) values when calculating a trajectory. The ballistic coefficient (BC) of a body is a measure of its ability to overcome air resistance in flight. You’ve probably seen that G7 values are numerically lower than G1 values for the same bullet (typically). But that doesn’t mean you should select a G1 value simply because it is higher.
Some readers are not quite sure about the difference between G1 and G7 models. One forum member wrote us: “I went on the JBM Ballistics website to use the web-based Trajectory Calculator and when I got to the part that gives you a choice to choose between G1 and G7 BC, I was stumped. What determines how, or which one to use?”
The simple answer is the G1 value normally works better for shorter flat-based bullets, while the G7 value should work better for longer, boat-tailed bullets.
G1 vs. G7 Ballistic Coefficients — Which Is Right for You?
G1 and G7 refer both refer to aerodynamic drag models based on particular “standard projectile” shapes. The G1 shape looks like a flat-based bullet. The G7 shape is quite different, and better approximates the geometry of a modern long-range bullet. So, when choosing your drag model, G1 is preferable for flat-based bullets, while G7 is ordinarily a “better fit” for longer, boat-tailed bullets.
Drag Models — G7 is better than G1 for Long-Range Bullets
Many ballistics programs still offer only the default G1 drag model. Bryan Litz, author of Applied Ballistics for Long Range Shooting, believes the G7 standard is preferrable for long-range, low-drag bullets: “Part of the reason there is so much ‘slop’ in advertised BCs is because they’re referenced to the G1 standard which is very speed sensitive. The G7 standard is more appropriate for long range bullets. Here’s the results of my testing on two low-drag, long-range boat-tail bullets, so you can see how the G1 and G7 Ballistic coefficients compare:
G1 BCs, averaged between 1500 fps and 3000 fps:
Berger 180 VLD: 0.659 lb/in²
JLK 180: 0.645 lb/in²
The reason the BC for the JLK is less is mostly because the meplat was significantly larger on the particular lot that I tested (0.075″ vs 0.059″; see attached drawings).
For bullets like these, it’s much better to use the G7 standard. The following BCs are referenced to the G7 standard, and are constant for all speeds.
Many modern ballistics programs, including the free online JBM Ballistics Program, are able to use BCs referenced to G7 standards. When available, these BCs are more appropriate for long range bullets, according to Bryan.
[Editor’s NOTE: BCs are normally reported simply as an 0.XXX number. The lb/in² tag applies to all BCs, but is commonly left off for simplicity.]
This article is copyright 2023 AccurateShooter.com. No 3rd Party republication of this article is allowed without advance approval and payment of licensing fees.
U.S. Supreme Court Invalidates ATF Rule Classifying Bump Stock Rifles as Machineguns
In the Garland v. Cargill case, the U.S. Supreme Court has ruled 6-3 that a semiautomatic rifle equipped with a bump stock is NOT a machinegun, and that the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) exceeded its authority by issuing a rule that classified the device as a machinegun. Accordingly the Supreme Court has determined the ATF’s bump stock rule to be void and invalid.
SAF Hails U.S. Supreme Court Ruling on Bump Stocks: “ATF Can’t Rewrite Law”
“This is a significant victory for gun owners because it reminds the ATF it simply cannot rewrite federal law,” said Second Amendment Foundation (SAF) Executive VP Alan M. Gottlieb. “The agency has just been reminded that it can only enforce the law, not usurp the authority of Congress.”
Writing for the majority, Associate Justice Clarence Thomas observed, “We hold that a semiautomatic rifle equipped with a bump stock is not a ‘machinegun’ because it cannot fire more than one shot ‘by a single function of the trigger.’ And, even if it could, it would not do so ‘automatically’ ATF therefore exceeded its statutory authority by issuing a Rule that classifies bump stocks as machineguns.”
For many years, the court ruling notes, ATF “took the position that semiautomatic rifles equipped with bump stocks were not machineguns” under the law. The agency “abruptly reversed course” in response to the mass shooting in Las Vegas in October 2017. The agency subsequently ordered bump stock owners to surrender them or destroy them within 90 days.
“Today’s Supreme Court decision demonstrates that it is impermissible for executive agencies to rewrite the law,” said SAF Executive Director Adam Kraut. “ATF exceeded its statutory authority by issuing a rule that was logically inconsistent with the plain text of the statute and cut into the prerogative of Congress. As the executive branch has continued to use ATF to implement its will and circumvent congressional authority, we are optimistic that today’s decision will send a message that such actions will not be tolerated and that the courts will strike down more regulations inconsistent with the law as Congress wrote.”
About the Second Amendment Foundation
The Second Amendment Foundation (SAF.org) is the nation’s oldest and largest tax-exempt education, research, publishing and legal action group focusing on the Constitutional right and heritage to privately own and possess firearms. Founded in 1974, SAF has grown to more than 720,000 members.
Lapua now offers a FREE Ballistics App. This was the first mobile ballistics App utilizing the 6DOF calculation model, making it one of most accurate ballistics Apps on the market. With this free mobile App you can calculate trajectories, range, bullet drop, turret adjustments and more.
Lapua’s sophisticated FREE Ballistics App has many great features — much more than you’d expect for a free App. If you do much shooting past 300 yards, or use a wide variety of bullets and/or cartridge types, definitely download the App and give it a try. For more details, read the Lapua Ballistics App User Manual. This handy PDF file explains how to set up the App and utilize all its powerful features.
Utilizes 6DOF, the most accurate calculation method
Always available with latest Lapua cartridge / bullet information
Quick and easy to change inputs like distance, wind speed and angle
Several result outputs available, with numerical, reticle, table and graph views
Create and clone your own firearm / reticle combinations
Choose between metric and imperial values
Set Point Blank-range to different sight-in distances and impact windows
Set Aiming point to moving target
Add custom bullets (calculations based on BC G1 or G7 and Siacci method)
Features include Sight-in-POI, Coriolis, Max Range and Cant Angle calculation
Compare up to 3 Lapua cartridge/bullet performances to each other
Custom trajectory tables
After downloading the FREE Lapua Ballistics App for Android device or Apple iPhone/iPad, you’ll want to access the Lapua Ballistics App Guide which shows how to use all the features and options. In addition, Lapua offers a handy FAQ Page with User Tips and Answers to frequently Asked Questions. After downloading the FREE App, you should definitely go to the FAQ page. It has helpful illustrations showing how to set up the App for your firearms and cartridge types.
Watch Video for Explanation of Lapua Ballistics App Features
Yet another .300 Blackout disaster. Unfortunately, that .300 Blackout cartridge can fit in a .223 Rem chamber. Shooting a .308-caliber bullet in .223 bore is a recipe for disaster.
The .300 AAC Blackout aka “300 BLK”, is a compact 30-caliber cartridge designed to work in AR-15 rifles. It has a shorter cartridge case to accommodate the bigger 30-caliber bullet while still fitting in a standard AR-15 magazine. Unfortunately, that’s the danger. A careless shooter can toss a .300 Blackout cartridge in with .223 Rem rounds without noting. And because the case-head size is the same as the .223 Rem (5.56×45) the rifle’s bolt assembly will happily chamber and fire the .300 BLK round. Problem is, that forces a .308 diameter bullet down an undersized .223-caliber bore. Not good!
This images were provided by Tactical Rifle Shooters on Facebook. The message was clear: “Don’t try to run 300 Blackout in your .223/5.56mm. It won’t end well. The problem is identical rifles and identical magazines but different calibers.”
Image from Accurate Shooter Forum. Cutaway shows the jammed .30-Cal bullet:
For those who MUST have a .300 Blackout, here are some things you can do:
1. Use different colored magazines for .300 Blackout vs. .223 Rem.
2. Fit all your uppers with caliber-labeled ejection port covers.
3. Mark .223 Rem upper handguards with the caliber in bright paint.
4. Mark all .300 BLK Rounds with heavy black marker.
Comments by Folks Who Viewed these .300 Blackout Disaster Photos:
“The .300 Blackout is simply a badly-designed round. A properly-designed round would have had a feature in the shape that would have prevented cross loading in the first place.” — D. Santiago
“I almost made that mistake… I had a magazine of 300 BLK inserted in my .223/5.56 all night. Fortunately, I never pulled the trigger. Once I realized the mistake, I almost got ill. [After that incident] I no longer own a 300 BLK.” — B. Welch
“Happened to me hog hunting from a helo. Gun exploded in my face.” — B. Hood
“Fire-forming projectiles [is] so wrong in centerfire!” — M. Stres
“Had some dude come into the store the other day wanting .300 Blackout ammo to shoot in his 5.56 AR. It took 15 minutes of explaining for him to understand you got to have a .300 Blackout Upper!” — R. Williams
Effects Of Cartridge Over All Length (COAL) And Cartridge Base To Ogive (CBTO) – Part 2
The way ambient temperatures, barrel heating, and powder warming can affect cartridge pressures (and hence velocities) was covered in a study Pressure Factors: How Temperature, Powder, and Primer Affect Pressure by Denton Bramwell. In that article, the author used a pressure trace instrument to analyze how temperature affects ammo performance. Bramwell’s tests yielded some fascinating results.
The better, up-to-date ballistics programs let you select either G1 or G7 Ballistic Coefficient (BC) values when calculating a trajectory. The ballistic coefficient (BC) of a body is a measure of its ability to overcome air resistance in flight. You’ve probably seen that G7 values are numerically lower than G1 values for the same bullet (typically). But that doesn’t mean you should select a G1 value simply because it is higher.
Drag Models — G7 is better than G1 for Long-Range Bullets
The 
