OVERBORE CARTRIDGES
Big case, small calibre – what’s the problem?
Ratio of powder-volume to bore-area: a case for concern?
SOME YEAR SA GO, while in the USA I made an opportunistic purchase of a Ruger No 1 rifle. It was one of those impulsive decisions that are so hard to resist. To my mind the smooth operation of the Ruger No 1’s single-shot actio n is a mong the most pleasing and comfortable of all rifle styles. It is reminiscent of the classic Farquharson action with some modern, wellengineered improvements. The rifle must be brought up into aim by a hunter to fully appreciate the light handling and its many other virtues.
It may seem to some that a singleshot action is a shortcoming but this is not so for most classes of meat hunting. In fact, it is surprising how quickly a second shot can be loaded and fired if necessary. This is because of the fast, unrestricted direct access to the chamber that many falling-block actions afford the shooter. The rifle handed to me was chambered for the high-velocity .25-06 cartridge introduced by Remington in 1969, based on the prevalent .30-06 Springfield case necked down to hold a 0.257ꞌꞌ bullet. I had it in mind to up-barrel the piece to a more suitable .30 calibre or similar should I find it unsuitable for the impala and warthog that keep us in meat. However, after using a highquality bullet in the smaller calibre, I decided to leave the rifle as is. It soon became apparent that it suited its intended employment admirably and dropped game like a stone.
But this large cartridge with its small bullet drew my attention to a perplexity regarding the term ‘overbore’ and the reported detriment to barrel life overbore cartridges are said to cause. Overbore is a ratio of the cartridge case volume, in other words its powdercarrying capacity, to the area of the barrel bore. Note that the area figure should not be confused with the calibre of the bore. Thus a large overbore suggests a large charge in relation to the cross-sectional area of the mouth of the cartridge case and accordingly, the size of a relatively small bullet for the big cartridge. Conversely, it may suggest under-filled cartridge cases when conforming to more conventional charges for the smaller bullet.
Volume case Overbore =
Area bore
The figure is usually expressed in the metric system with the volume in millilitres of water, conveniently the same as grams of water, and the area of the bore in square centimetres. There are other measurement methods. One table given on an internet site
expresses the volume of a case by the weight of powder in grains needed to fill it against the bore area in square inches. With this in mind, a useful figure to have at hand is the density of most nitrocellulose propellants, being about 1.6 grams or 24.7 grains per cubic centimetre. A list using this powder-fill method with no mention of the powder designation, shows a .458 Winchester with a moderate ratio of 570, and my .25-06 Remington with a ratio of 1 272 which, along with numerous other calibres, is classed as excessive. Both methods reveal the .25-06 to be clearly overbore.
The significance of this observation is that small calibres with large charges will tend to wear out barrels more quickly. This is mostly due to excessive and concentrated heat at the start of the bore. However, overbore numbers are generalisations. They do not always reflect the complete picture. In fact I believe the .25-06 to be an excellent calibre when using several moderate load options to avoid causing undue harm to the barrel. While there are other, lesser ballistic contributors to barrel wear, some beliefs are totally erroneous. For example it has been said that in high-velocity overbore cartridges, up to one third of the available energy is spent accelerating the rotation of the bullet from its state of rest to the hundreds of thousands of RPM at the muzzle. This is attributed to a supposed large frictional burden and also overcoming the inertia of the high-velocity bullet. It is not true. One wonders where such a belief originated. The required energy to do this is much less. Unfortunately specific figures are difficult to give because of the many influences that must be incorporated in an elaborate calculation. The following are the more important offenders that shorten barrel-life and which should be taken into account when developing all loads and especially those for cartridges that are considered to be in the overbore category.
Because the burning temperatures of nitrocellulose propellants lie in a range from 2 560°C to 3 300°C, an intense
flame with plenty of available heat can cause the surface in the barrel just beyond the chamber to rise to 1 000°C or more and this may last for about twenty milliseconds. Fast powders tend to burn at the upper end of this temperature range because they release their heat more quickly. But it is the availability of the energy in the form of heat as opposed to the temperature which is the prime consideration when estimating barrel life. Different sized charges deliver different amounts of energy in the form of hot gas. Increasing the charge and thus the amount of gas will not only produce more pressure, it will have more kilojoules of heat to release.
The transfer of an excessive amount of heat into the barrel steel at the lead into the bore will result in erosion. It can also change the metallurgy and cause micro-cracking and hydrogen embrittlement. Interestingly, the risk of hydrogen embrittlement is a good reason for not allowing gun oil to remain in the bore when shooting. In the case of overbore cartridges, the heat damage is much exacerbated by the concentration of a relatively large flame exiting through the small cartridge mouth. This is why overbore ratios relate to the area of this troublesome zone. The smaller the area at the mouth, the more concentrated the jet of hot of gas will be. Understandably, this draws the attention of critics of overbore cartridges to the .25-06 which has 30% less area at the cartridge mouth than its .30-06 parent.
And then there is the lesser and yet still important problem of friction to consider. Most loads intended for overbore calibres are capable of producing very high velocities. The article “The Life of Barrels” in the August 2012 edition, mentioned the possible damage through friction. It is worth repeating. Harm through abrasive wear in a barrel does not rise in proportion to velocity. It is likely to rise exponentially when approaching three thousand feet per second and beyond. By including this with the larger damage caused by heat erosion at the entrance to the bore, one opinion concludes barrels should have certain life expectancies. (See table.)
While the estimate seems to have no foundation of statistical data to support it, it does point to the adverse effect high velocities and flame damage will have on a barrel.
NOW LET ME hasten to the defence of my .25-06 Remington. The figures for the expected barrel life given in the table do not take into account how the rifle is used. Sustained rapid fire and lengthy plinking sessions at the range may cause hot-spot temperatures in the bore to rise to harmful levels. On the other hand, my occasional scope zeroing-in sessions and perhaps a new load development along with a hunt or two during the course of a year are much less likely to do any harm. Also, the flatter trajectories and higher energies offered by velocities beyond 2 800fps are superfluous to my modest hunting needs. For this reason I keep my .25-06 loads to a more expedient 2 800fps. But let us presuppose a worst-case scenario where a habitual and frequent use of potent ammunition which exploits the maximum energy potential of the .25-06 cartridge causes the barrel life to shorten to 1 500 rounds as shown in the table. Presumably this would transpire after consuming about 75 boxes of 20 rounds each. This is still quite a lot for a hunting rifle. My Ruger No 1 is not a plinking rifle. Neither do I intend to use it as an ultra-high velocity piece other than having the beneficial option of using hot loads should long point-blank ranges be infrequently required.
So how long does it take to use the expected 5 000 rounds for my preferred velocity? Although often used and enjoyed, it is similar to other hunting calibres in my battery. Fifty rounds a year is a generous reckoning so the estimate should be about one hundred years. I am happy with this.
Finally, there is the question of an under-filled case. My most frequently used load is 47gr of S365 behind a 120gr bullet to achieve the desired 2 800fps. In this event the cartridge case is 80% full. Clearly this is not a problem.
Overbore Cartridges in Order of Progressive Severity:
■ .270 Winchester
■ .250 Ackley
■ 7mm Rem SAUM*
■ .243 Winchester
■ 6.5-284 Norma
■ .220 Swift
■ .22-250 Remington
■ .25-06 Remington
■ 7mm Rem Mag
*Short Magazine Ultra Mag
The merits or otherwise of different calibres will always be a subject for debate. But in the course of such discussions, a few may be unfairly criticised through misinformation and hearsay. In my view, the .25-06 Remington is an excellent cartridge fit for the yearly procurement of meat.