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Barrel Movement- Part 1

One of the consistent goals for any shooter when reloading brass is to develop the most consistently accurate load possible. On the surface, the task appears daunting as one seems to be faced with having to trying hundreds of combinations in different loading manuals to find one that works well in your particular firearm. However, as we will see, there is an easier way.

In this multi-part series, we will be looking at barrel movement and how our loads should take into consideration even the slightest movement of the barrel.

First a little theory

Every barrel develops harmonic vibrations when a cartridge is fired, similar to a string being shaken. A gun barrel's vibration can be described as a 3-dimensional sine wave, or corkscrew movement and is caused when the bullet is accelerated into a rapid spin by the rifling.

It is impossible to entirely eliminate barrel movement. Even a thick barrel's muzzle will move with every shot and any velocity variation will alter where the muzzle is located as the bullet departs. This random movement of the muzzle gives rise to increased group size. Most experienced gun builders agree it is best to allow a barrel to flex. The idea is if the barrel's movement can't be eliminated the next best thing is to load rounds in such a manner that causes these vibrations to be consistent and predictable. That's why well-made guns will often have their actions, and only the first inch or so of the rear of the barrel, bedded tightly into the stock to hold the receiver firmly, with the remainder of the barrel free floated. In addition, short, thick barrels have wide(r) nodes so velocity isn't as critical to achieving a sweet spot.

Since the velocity of the bullet passing through the barrel affects the way it flexes, accurate loads should deliver as consistent a velocity from shot to shot as is possible so that the bullet exits the muzzle at the same point in the "flex.". You can control this to a degree but it is impossible to entirely eliminate shot to shot velocity deviation. At around a variation of 10 to 12 f/s it may become almost impossible to reduce the effect any further.

Finding the “Sweet Spot”

It has long been understood that barrels perform best within certain velocity ranges. These velocity ranges are commonly referred to as "harmonic nodes,” with the less technical name being "sweet spots". The reason for this is that the tensile strength of the metal alloy increases as it moves further away from its static state. The barrel gets stiffer when it is forced to the extremity of its movement. At the point of maximum movement, slight velocity variations change the muzzle location less; resulting in lower shot dispersion and thus a smaller group size. What most shooters don't understand is the harmonic vibration is related to the mass of the bullet. Therefore, once the harmonic node(s) for a given weight bullet is identified, a lot can be learned, if you know the velocity.

There is also a new theory of "barrel timing" being developed based upon data obtained from strain gauges. Upon firing, the chamber swells slightly and an annular ring of expansion travels down the barrel causing the bore to expand slightly and this effect continues as the expansion reflects back and forth along the barrel diminishing with each passage, similar to the ripple in water from throwing a stone. Initial data suggests that not only should a load perform best at one of the velocity nodes but that the bullet should not exit the muzzle at the same time that the expansion ring reaches the muzzle as the slight increase in bore size adversely affects accuracy.

In part 2 of this series, we will begin to look at load development and how to take barrel movement into consideration.