September 28th, 2018

“Overbore” — A Practical Definition by the Numbers

Overbore cartridge chart index ratio 50 BMG Case Capacity 30Br

What is “Overbore”? That’s a question rifle shooters can debate to no end. This article from our archives proposes one way to identify “overbore cartridges”. We think the approach outlined here is quite useful, but we know that there are other ways to define cartridges with “overbore” properties. Whenever we run this article, it stimulates a healthy debate among our readers — and that is probably a good thing.

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

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, while not perfect, 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.”

Permalink Bullets, Brass, Ammo, Tech Tip 1 Comment »
September 28th, 2018

Learn How the Human Ear Works — And Protect Your Hearing

hearing protection inner ear anatomy science hearing medical electronic muffs earplugs

hearing protectionAll shooters, even rimfire enthusiasts, should always wear ear protection when at the range. A typical rifle gunshot is very loud — in the region of 140 to 170 decibels (the pain threshold is 130-140 db). Without ear protection, you can permanently damage your hearing during a single shooting session. We all know older shooters who are partially deaf, or who suffer from Tinnitus, because they didn’t use earplugs or muffs when they were younger.

How Humans Hear Sounds — Amazing Video Reveals All
The human sense of hearing involves multiple delicate internal membranes, bones, organs, and nerves. Shooters understand the importance of protecting their hearing, but they may not understand the bio-mechanics of human hearing. We hear sounds through Auditory Transduction. Sound waves vibrate the ear drum (tympanic membrane), but that is only the beginning. These vibrations are passed along via tiny rocker-arm-like bones to be “processed” in a spiral chamber, the cochlea.

This remarkable VIDEO explains how humans hear sounds. We strongly recommend you take the time to watch and learn. The hearing you save may be your own!

Click Speaker Icon to turn on the video’s soundtrack.

Vibrations moving through the cochlea are separated into frequencies and then sent as neural messages to the brain. It is an astonishingly complex process, one that truly seems miraculous when you examine the bio-engineering involved. In the Video above, the process of human Auditory Transduction is explained and illustrated with 3D animation. You really should watch this amazing video. By the end you will have a new-found appreciation for your ability to hear.

hearing protection inner ear anatomy science hearing medical electronic muffs earplugs

Every shooter should own a pair of Electronic muffs, even if you prefer shooting with earplugs and/or standard muffs. Electronic muffs are great when you are doing spotting duties or are working near the firing line. They let you hear ordinary conversations while still providing vital hearing protection. You can also wear plugs under muffs for extra sound attenuation.

Ear diagram courtesy Siemens Medical Solutions.

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September 28th, 2018

How to Clear-Coat Laminated Wood Stocks

Laminated wood stocks offer an excellent combination of price and performance, and they can be obtained in a myriad of styles to suit your discipline — hunting, benchrest, tactical, silhouette, or high power. Laminated stocks can be a little trickier to finish compared to a hardwood such as walnut, as laminates are often delivered in bright or highly contrasting colors. Traditional wood finishes can alter the colors. Also, filling the pores in laminated stocks is an issue.

Automotive clear-coat products have become popular for finishing laminated wood stocks because they won’t alter the stock’s colors, and the clear-coat provides a durable weather-resistant finish. Clear-coat is also easy to “touch up” and it fills pores better than some other alternatives. Mike Ricklefs has written a comprehensive article on stock painting that includes a special section on clear-coating over laminated woods. If you want to clear-coat a stock, Mike’s article is a must-read!

In that Stock Painting Article, Mike offers these tips:

1. When finishing laminated stocks with clear-coat, you need to prepare the wood carefully, and build up quite a few thin layers one at a time. Begin by sanding, with progressively finer paper, all the way to 400 grit. Certain laminated stocks are so rough when they come from the stock-maker, that you may have to be very aggressive at first. But be careful with angles and the edges of flats. You don’t want to round these off as you sand.

2. After sanding, use compressed air to blow out all dust from the pores of the wood. This is very important to avoid a “muddy” looking finish. If you don’t blow the dust out with air before spraying the clear it will migrate out as you apply the clear. Also, after each sanding session, clean your painting area to remove excess dust. I also wet down the floor of my spray booth to keep the dust down.

3. Some painters recommended using a filler to close the pores. That’s one technique, but the filler can detract from the clarity of the final finish. Rather than use a pore-filling sealer, I use a high solids or “build” clear for the initial applications. This is slightly thicker than “finish” clear and does a good job of sealing the pores. Three (3) fairly heavy coats of “build” clear are applied. If you get a thick spot or a run in the finish at this point, it is not the end of the world but this does create more sanding work.”

There is a helpful thread in our Shooters’ Forum that discusses the use of clear-coating on laminated stocks. Member BHoges offered this advice: “Stick with Diamont, Glassurit, and Spies. If anyone has questions, I painted cars for a long time.”

Clear-coat Laminated woodForum member Preacher, whose bolt-action pistol is shown at right, states: “I buy my two-part clear-coat from the local NAPA dealer. They recommended Crossfire mixed 4:1. I really like the end results. There are six coats on that stock that were sanded down to bare wood for the first two, and then 600 wet-sanded for the other four coats. Two to three coats would be sufficient if the pores were filled first, but I would rather fill ‘em with the clear as it seems to make it appear deeper and I have the time to devote to it. I have PPG’s Deltron DC 3000 clear-coat on a few stocks of mine, but I like the NAPA better price wise, and it seems to hold up just as good as the Deltron.”

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