Long-Range Shooting by Sean I’Ons

  • Friday, 07 September 2018 10:05
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Long-Range Shooting by Sean I’Ons: The rifle – called the “platform” in the jargon

Last month we discussed cartridges suitable for long-range shooting. Now it’s time to choose a rifle. You can, of  course, use any off-the-shelf rifle, but in  serious competition you will be  handicapped if the bore’s twist rate is not suited to the high-BC bullets designed for long-range application, and you may encounter other limiting factors.

Long-range rifle options differ widely according to personal preference, budget and application, but the one common requirement is a high level of consistently repeatable accuracy throughout a wide spectrum of conditions. To achieve this, several factors must come together in harmony – some platform-related, others cart- ridge-related. Rigidity and stability in the barrel, receiver and stock/chassis are imperative. The stock must be designed to provide rigid and stable support to the receiver and barrel in all conditions. Secondary to this, ancillary equipment is sometimes attached to the stock, and a dedicated L-R chassis facilitates this far better than other stock types.

Barrel Selection

L-R barrel profile specs vary, but the primary categories are Match (Straight Tapers), Varmint or Palma (Compound Tapers) – all on the heavy side. The longer the barrel you choose, the heavier (thicker) it needs to be to dampen and control vibrations and oscillations (there it is again – rigidity) caused by the bullet’s passage through the bore. The bullet’s flight path is influenced by the oscillations of the muzzle at the instant of the projectile’s exit. 

A bullet accelerates from stationary to 3 000fps in a 24" barrel in 1.1 milliseconds. Where the accurate life of a barrel lasts for about 3 000 shots, this adds up to an actual working lifespan of only 3.3 seconds. 

Some insight as to what takes place during the firing of a cartridge and a bullet’s accelerated passage through the bore is provided by a study carried out by Chris Long, who calculated the frequency of the shock waves travelling the length of the barrel and being reflected back on reaching the muzzle. The reflected shock waves superimpose on each other, providing a series of expansions and contractions running up and down the barrel, much like a snake swallowing an egg which then moves back and forth within the length of the snake. To ensure repeatable flight paths, every bullet should exit the muzzle during a window of consistency. 

The speed of these shock waves was found to vary according to the length of the barrel from bolt-face to muzzle-crown (excluding the suppressor/brake if fitted). The results indic- ate an optimal barrel time (OBT) and series of nodes which are consistent with the calculated consistency windows. (In this context, a node is a point of minimum disturbance in a standing wave system.) The theory is that if you tailor the load to have a barrel time (BT) consistent with the node, it will result in best accuracy from a given platform. This has been tested and shows an extremely high correlation in practical applications and is widely used by reloaders to develop and refine loads for their rifles.

Read the full article in the October 2018 issue of Magnum.

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