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