Defining “Overbore” Cartridges via Comparative Index

Forum Member John L. has been intrigued by the question of “overbore” cartridges. People generally agree that overbore designs can be “barrel burners”, but is there a way to predict barrel life based on how radically a case is “overbore”? John notes that there is no generally accepted definition of “overbore”. Based on analyses of a wide variety of cartridges, John hoped to create a comparative index to determine whether a cartridge is more or less “overbore”. This, in turn, might help us predict barrel life and maybe even predict the cartridge’s accuracy potential.

John tells us: “I have read countless discussions about overbore cartridges for years. There seemed to be some widely accepted, general rules of thumb as to what makes a case ‘overbore’. In the simplest terms, a very big case pushing a relatively small diameter bullet is acknowledged as the classic overbore design. But it’s not just large powder capacity that creates an overbore situation–it is the relationship between powder capacity and barrel bore diameter. Looking at those two factors, we can express the ‘Overbore Index’ as a mathematical formula — the case capacity in grains of water divided by the area (in square inches) of the bore cross-section. This gives us an Index which lets us compare various cartridge designs.”

OVERBORE INDEX Chart

So what do these numbers mean? John says: “My own conclusion from much reading and analysis is that cartridges with case volume to bore area ratio less than 900 are most likely easy on barrels and those greater than 1000 are hard on barrels.” John acknowledges, however, that these numbers are just for comparison purposes. One can’t simply use the Index number, by itself, to predict barrel life. For example, one cannot conclude that a 600 Index number cartridge will necessarily give twice the barrel life of a 1200 Index cartridge. However, John says, a lower index number “seems to be a good predictor of barrel life”.

John’s system does give us a benchmark to compare various cartridge designs. If, for example, you’re trying to decide between a 6.5-284 and a 260 Remington, it makes sense to compare the “Overbore Index” number for both cartridges. Then, of course, you have to consider other factors such as powder type, pressure, velocity, bullet weight, and barrel hardness.

Overbore Cases and Accuracy
Barrel life may not be the only thing predicted by the ratio of powder capacity to bore cross-section area. John thinks that if we look at our most accurate cartridges, such as the 6 PPC, and 30 BR, there’s some indication that lower Index numbers are associated with greater inherent accuracy. This is only a theory. John notes: “While I do not have the facilities to validate the hypothesis that the case capacity to bore area ratio is a good predictor of accuracy — along with other well-known factors — it seems to be one important factor.”

OnTarget Program Measures Group Sizes

Jeffrey Block has created a great new FREE software program, OnTarget, that measures shot groups quickly and precisely. All you need is a photo or scan of your target. The program allows you to set your target distance, and provides caliber-specific tools to precisely mark the center of each shot. Once you’ve marked each bullet hole, Jeff’s OnTarget program automatically calculates group center, maximum group spread (CTC), average distance to center, group width and height, and group offset from point of aim. The program will even measure multiple groups on the same target.

After just a few minutes spent learning the program’s tool buttons, we were able to plot shot groups on a variety of targets with ease.

Once you select the target distance and bullet diameter, figuring group size is a simple matter of centering a circle tool over each bullet hole. Then the program “connects the dots” and provides all the info you could want automatically.

The program worked with bullet holes as small as 17 caliber and as large as 50 caliber. It is very precise, but remember that if your target photo was taken at an angle, distorted perspective can cause slight errors in measurement. Therefore, for the ultimate precision, you want to start with a flat scan of the target.

OnTarget Compared to Measuring Manually
We found OnTarget to be especially useful for groups with widely dispersed bullet holes, or very small bullet holes, such as 17 caliber holes. We’ve found that it’s difficult to measure 17-cal group sizes with a standard caliper, because the tool itself obscures the tiny holes. With OnTarget, the program can zoom up your target view, making it much easier to plot the center of each shot. And with a widely dispersed group of shots, the program automatically finds the two most distant shots. You can’t mistakenly pick the wrong pair of shots to measure.

Flash Tutorial Shows How It Works
Jeff created an excellent animated Flash tutorial demonstrating OnTarget’s functions. It shows how to import a target image or scan, how to set target distance and scale, how to set bullet size and circle each bullet hole, and how to save the marked and measured target.

CLICK HERE to VIEW TUTORIAL

MEASURING REAL TARGETS

Here are examples we created with OnTarget. The first photo shows a 17 Mach 2 target. These tiny 17-cal holes are notoriously hard to measure. With OnTarget, it’s a snap. You just load the target image into the program, zoom in with the controls, and then click on the center of the holes. The program automatically calculates group size, displaying measurements in both inches and minutes of angle (MOA)

Original Target (with ruler for scale)

Target Captured and Displayed in Program

Detail of Group, Enlarged by Program

10-shot Groups? — No Problem
Here’s another target, showing 6mm bullet holes at 600 yards. The first image shows the target image loaded into the program with the ten holes circled in red.

Target Displayed in Program

For this target we have used the Aiming Point option. The Aim Point was set at the center of the “X” and the program calculates average distance from the Aim Point. Very cool.

Detail of 10-Shot Group, Enlarged by Program

No Scanner Needed
The OnTarget program grabs target scans directly from a flatbed scanner using Microsoft’s Windows Image Acquisition system. But don’t worry if you don’t have a scanner. You can just take a digital photo of your target and OnTarget will import it quickly and easily. To set target scale, a simple tool allows you to mark a known length on the target (such as the diameter of the “X” Ring), and the program will then size the target accordingly.

Bottom Line — Great Program, Download It Today
Jeffrey Block has done a great service for shooters by creating the FREE OnTarget program. It is easy to learn, it functions great, and it can save you time and effort measuring targets. It also lets you easily archive and compare multiple targets produced during load development or rifle testing. You can record ammo type, date, location, weather etc. in note fields accessed by “Group Info” and “Target Info” tabs.

Keep in mind that OnTarget was NOT created to replace existing methods for scoring competition targets. But for all other target measuring purposes it does a great job. You should visit Jeff’s website, OnTargetShooting.com, view the tutorial, and check out OnTarget for yourselves.

Hoppe Offers Dasher Tuning Tips

The 6mm Dasher is based on the 6mm BR cartridge with the shoulder blown forward about 0.100″ and “improved” to 40°. Case capacity is raised to about 41.0 grains. This allows the Dasher to drive 105-108gr bullets comfortably at 2970-3000 fps without over-stressing the brass.

Robert Hoppe is one of the top 600-yard shooters in the country. Last fall, shooting a 6 Dasher, Robert nailed a 0.5823″, 5-shot group. At the time it was the smallest group ever shot in 600-yard registered benchrest competition. Just last weekend, John Lewis shot even smaller with an IBS Heavy Gun, but Robert’s 0.5823″ still remains the NBRSA 600-yard record, and the smallest group ever shot at 600 by a 17-lb class rifle. Robert has been very successful in the 600-yard game, and he finished 5th recently at the NBRSA 600-yard competition. He knows how to wring the best accuracy out of the 6mm Dasher cartridge. Here Robert offers some tips on load development and tuning for the 6mm Dasher.

CLICK HERE to watch HOPPE VIDEO

Tilley’s 260 Ackley Improved

Forum member Ron Tilley finished fourth overall at the recent NBRSA 600-Yard Nationals. This was an impressive performance for a relative newcomer to the 600-yard game without “local knowledge” of the Sacramento range. The top 3 finishers, Jerry Tierney, Peter White, and Craig St. Claire, all have extensive experience shooting at Sacramento.

While many of the top shooters were using a 6mm in Light Gun (such as a 6 Dasher or 6-6.5×47), Ron campaigned a 260 Ackley Improved. This is based on a 260 Rem case, with the shoulder blown forward to 40 degrees. Improving the 260 Rem case adds just enough extra capacity to get the 140gr-class bullets into the 2900+ fps velocity window where they seem to perform best.

Ron starts with Lapua 243 cases, necked up to 6.5 mm. He was shooting 140gr Berger VLD bullets, pushed by Hodgdon H4831sc powder and Russian primers. Ron says this combination has been “very accurate right out of the gate.” Ron’s 2920 fps load delivers 5-shot groups in the high teens and low twos, with very low ES and SD. In the video interview below, Ron explains the merits of the 260 AI cartridge, and offers tips on case prep and loading for accuracy.

CLICK HERE to WATCH TILLEY VIDEO

Ron was shooting an eye-catching rifle. It featured a carbon fiber and wood Terry Leonard stock, BAT Action, and Nightforce scope. With all-premium components, Terry’s 260AI is a beautiful rig that shoots as good as it looks.

Forming Dasher Cases with Pistol Powder

Forum member Skeeter has put together a new 6mm Dasher falling block varmint rifle. The Dasher case is based on the 6mm BR Norma cartridge with the shoulder blown forward about 0.100″ and out to 40°. This gives the Dasher roughly 3.5 grains added capacity compared to the standard 6BR.

Skeeter needed to form 300 cases for an upcoming varmint holiday. Skeeter decided to fire-form his brass without bullets. This method avoids barrel wear* and saves on components. There are various ways to do this, but Skeeter chose a method using pistol/shotgun powder, some tissue to hold the powder in place, Cream of Wheat filled to within an 1/8″ of top of the neck, and a “plug” of tissue paper to hold it all in place. Shown below are cases filled with a pistol/shotgun powder charge topped with Cream of Wheat and then a tissue paper plug.

To ensure the case headspaced firmly in his Dasher chamber, Skeeter created a “false shoulder” where the new neck-shoulder junction would be after fire-forming. After chamfering his case mouths, Skeeter necked up all his cases with a 0.257″ mandrel (one caliber oversized). Then he used a bushing neck-sizing die to bring the top half of the neck back down to 0.267″ to fit his 0.269″ chamber. The photo below shows how the false shoulder is created.

After creating the false shoulder, Skeeter chambered the cases in his rifle to ensure he could close the bolt and that he had a good “crush fit” on the false shoulder, ensuring proper headspace. All went well.

The next step was determining the optimal load of pistol powder. Among a variety of powders available, Skeeter chose Hodgdon Titewad as it is relatively inexpensive and burns clean. The goal was to find just the right amount of Titewad that would blow the shoulder forward sufficiently. Skeeter wanted to minimize the amount of powder used and work at a pressure that was safe for his falling block action.

Working incrementally, Skeeter started at 5.0 grains of Titewad, working up in 0.5 grain increments. As you can see, the 5.0 grain charge blew the shoulder forward, but left it a hemispherical shape. At about 7.0 grains of Titewad, the edge of the shoulder and case body was shaping up. Skeeter decided that 8.5 grains of Titewad was the “sweet spot”. He tried higher charges, but the shoulder didn’t really form up any better. It will take another firing or two, with a normal match load of rifle powder and a bullet seated, to really sharpen up the shoulders. Be sure to click on the “View Larger Image” link to get a good view of the cases.

The process proved to be a success. Skeeter now has hundreds of fire-formed Dasher cases and he hasn’t had to put one bullet through his nice, new match-grade barrel. The “bulletless” Cream of Wheat method allowed him to fire-form in a tight-necked barrel without neck-turning the brass first. The only step now remaining is to turn the newly Dasher-length necks down about .0025″ to fit his 0.269″ chamber. (To have no-turn necks he would need an 0.271″ or 0.272″ chamber).

Skeeter didn’t lose a single case: “As for the fire-forming loads, I had zero split cases and no signs of pressure in 325 cases fire-formed. Nor did I have any misfires or any that disbursed COW into the action of the firearm. So the COW method really worked out great for me and saved me a lot of money in powder and bullets.” To learn more about Skeeter’s fire-forming process, read this Dasher Fire-Forming Forum Thread.

*Skeeter did have a fire-forming barrel, but it was reamed with a .269 chamber like his 10-twist Krieger “good” barrel. If he fire-formed with bullets, he would have to turn all 300 necks to .267″ BEFORE fire-forming so that loaded rounds would fit in the chamber. Judging just how far to turn is problematic. There’s no need to turn the lower part of the neck that will eventually become shoulder–but how far down the neck to turn is the issue. By fire-forming without bullets now he only has to turn about half the original neck length, and he knows exactly how far to go.

RCBS APS Hand Priming Tool

Readers often ask: “What priming tool should I use?” There is no simple, clear-cut answer. Different tools have different advantages and disadvantages. Many short-range benchresters like the Sinclair one-at-a-time priming tool. They believe this unit has the best “feel” for seating primers.

Others prefer the K&M tool, another “single-loader”, because it can be fitted with a seating depth gauge, for the ultimate in precision. On the other hand, one top shooter with a couple National Championships under his belt favors the modest $19.00 Lee Autoprime tool: “It’s simple, efficient and has served me well for years–I’m not convinced the fancy, expensive tools do a better job.”

This editor has tried all of these tools, and personally I favor the RCBS APS hand priming tool. It has two important advantages. Number one, primers are held in color-coded plastic strips so you never have to touch the primers. If you buy the pre-loaded strips, there is no chance of getting an upside-down primer, and you never have to fool with flipping primers in a tray. Second, the APS tool has a “universal” shell-holder. This employs spring-loaded jaws so it can fit any size cartridge, from a 17 Fireball to large magnums. The unit has a comfortable grip and plenty of leverage. Each time you seat a primer, the strip automatically advances.

Currently, only CCI primers can be purchased pre-loaded into strips. But there is an inexpensive tool that allows you to load Federal, Winchester, or Wolf primers into the strips. The strips come in a variety of colors (Red, White, Blue, Orange, Yellow, and Black), so you can sort your primers by color.

The RCBS APS hand primer is ideal when doing primer comparison testing. You can slide one strip of 25 primers in the tool, seat 5 or 10, then easily remove the strip with the remaining primers, and slide in a second strip with another primer type. You can slip the first strip back in a storage box and the primers remain ready to use.

As contained in the strips, primers can also be used in an RCBS 2000 progressive press, a bench-mounted priming unit, and a special device that fits into the top of a Rockchucker or similar press. I have loaded over 20,000 pistol rounds with an RCBS progressive press and I have had not a single flipped primer. Not one. The strip-priming system also makes it easy to switch from small to large primers on the progressive–this can literally be done in under 20 seconds.

The bench-mounted unit is ideal for priming large quantities of cases. The long handle provides plenty of leverage, and you can work fast, as the primers automatically feed through the system. When I load large quantities of varmint rounds, I use the bench-mounted APS system.

Precise Case Trimming without a Micrometer

Do you have a Wilson case trimmer but can’t afford the $84.00 to add the handy Micrometer upgrade (Sinclair Int’l item 05-4500)? Well Boyd Allen has come up with a clever use of a standard set of calipers that lets you set the cut length precisely within .001″. Just open the jaws of your caliper and put one jaw on the outboard end of the ring (with set screw) that holds the threaded length-adjusting rod. Set the other caliper jaw on the flat face at the end of the threaded rod that contacts the case rim. If you have a sample case set to the correct cut length, use that to set the position of the threaded rod. Then use your calipers to measure that length. This way you can repeat the cut length each time, or adjust the trimmed case precisely in .001″ increments.

If you change from one cartridge to another, just use your calipers to re-set the desired cut length. Alternatively, ReedG notes you can use the inside caliper jaws and measure directly from the end of the threaded rod to the cutter face. That’s a bit trickier, but it measures actual trim length.

Hodgdon and ADI Powder Equivalents

Many of the most popular powders sold under the Hodgdon brand in the USA (including Varget and H4350) are made by ADI Ltd. in Australia. Some load manuals list ADI data, but not Hodgdon data, or vice-versa, so we’ve compiled this list of equivalent powders. If you can’t find a recommended load for a particular Hodgdon powder in your caliber, download the latest ADI Smokeless Powders Handloaders Guide, a 2.8 megabyte Acrobat file.

Here’s a list of ADI to Hodgdon Powder equivalents:



AS30N=Clays AP50N = (No Hodgdon) AS50N = International AP70N = Universal AP100 = (No Hodgdon) AR2205 = H4227 AR2207 = H4198 AR2219 = H322 Bench Mark1 = (No Hodgdon) Bench Mark2 = BenchMark

AR2206 = (No Hodgdon) AR2206H = H4895 AR2208 = Varget AR2209 = H4350 AR2213 = (Discontinued) AR2213SC = H4831 AR2217 = H1000 AR2225 = Retumbo AR2218 = H50BMG

Reloading Info on ShootingUSA Television

On today’s episode of ShootingUSA, RCBS factory expert Allan Jernigan demonstrates safe, basic reloading techniques for pistol, rifle, and shotgun. You may be an expert rifle reloader, but specialized techniques can help you load more consistent pistol ammo, on both single-stage and progressive presses. And, of course, shotshell reloading is a different game altogether — you have more components to deal with, and up to three separate stages just for the crimp. In this show, Jernigan offers tips on optimizing accuracy and explains how to save money on components — so you can maintain your reloading output even as the costs of bullets, shot, primers and powder are on the rise. Jernigan demonstrates reloading methods using the RCBS RockChucker, RCBS 2000 Progressive, and RCBS Grand shotshell loader. Also in this episode, Wheelgun wizard Jerry Miculek offers speed-shooting Pro Tips for revolver fans.

ShootingUSA airs Wednesday Nights on the Outdoor Channel:

Eastern Time - 4:30 PM, 8:30 PM, 2:00 AM (Thursday)
Central Time - 3:30 PM, 7:30 PM, 1:00 AM (Thursday)
Mountain Time - 2:30 PM, 6:30 PM, Midnight
Pacific Time - 1:30 PM, 5:30 PM, 11:00 PM (And Arizona)

Norma 203B Powder Now Available

Norma 203B has been nearly impossible to obtain in the USA in recent years. Expressly formulated for the 6mmBR cartridge, many consider the elusive Norma 203B powder to be the “Holy Grail” of propellants for this popular chambering. It also works well in cartridges for which Varget, VV N150, or Reloder 15 is suitable.

Now, with Black Hills Shooters Supply (BHSS) as a distributor, Norma is finally making 203B available again for American reloaders. Consumers can’t buy direct from BHSS, but retail vendors can place wholesale orders with BHSS, which has 472 pounds of Norma 203B as of 4/7/2008. Powder Valley has Norma 203B in stock right now at $19.00 per pound, and Grafs.com has it for $21.99 per pound (item NP203B1).

Industry scuttlebutt says that Norma 203B is the same as Alliant Reloder 15. While both are crafted by European powder-maker, Bofors, we’re told that 203B is not identical to Reloder 15. At Shot Show, this editor put the question directly to Norma’s CEO, Torb Lindskog: “Is Norma 203B the same as Reloder 15?” Lindskog answered: “The two powders are very similar, yes. But there are some differences–they are not exactly the same. We like to think our powder is more uniform.”

We’ve tried 1998 vintage Norma 203B in a 6BR and it was very similar to Reloder 15 in appearance, but the Alliant powder yielded slightly higher velocities. What we noticed shooting a 6BR with Norma 203B was a HUGE accuracy window. With 105gr bullets, the powder shot very well at a wide variety of charge weights, from mild to max. Plus, ES and SD were extremely low. Average 5-shot Extreme Spread was 8 fps, with three 5-shot groups measured. Norma 203B did display greater temp sensitivity than Varget, however. If you want to try Norma 203B for yourselves, don’t hesitate — call Powder Valley, (800) 227-4299, or other stocking retailer before the current supply is exhausted. It could be many months before the next shipment arrives.

6mmAR Case-Forming Video

A full-time lawyer, Robert Whitley is also an avid prone, Highpower, and cross-course shooter. Over the past two years, Robert has pioneered a cartridge for these disciplines (as well as long-range varminting) based on the 6.5mm Grendel necked down to 6mm. Robert calls the cartridge the “6mmAR”.

The 6mmAR cartridge has performed very well in AR-based comp guns, both with shorter bullets loaded to mag length, and 105-107gr bullets single-loaded. Robert has complete info about the 6mmAR on his website, 6mmAR.com, including recommended loads and field test results.

Though making 6mmAR brass from 6.5 Grendel brass is quite straight-forward and simple, Robert receives many questions about forming the brass, so he recently prepared a video. The two-minute video shows how quick and easy the brass-forming process can be:

Robert also builds and sells products for across-the-course shooters, including complete AR uppers, chambered in a variety of calibers, including 6mmAR. Jerry Tierney, 2005 NBRSA 1000-yard champion, has a 6mmAR upper built by Robert and Jerry says the upper “Absolutely shoots great. I love it.” In the past few months, Jerry has broken many NRA senior records with his new 6mmAR-chambered rifle.

CLICK HERE for Feature Article on 6mmAR and Robert Whitley (many photos)

CLICK HERE for 6mmAR LOAD DATA