There is an old “everyone knows” myth about barrels having “internal stresses” that can show up when the barrel heats by shifting the zero, or opening up groups. It’s a fine hypothesis in that it does explain the observed phenomena, however, it is wrong.
Barrels have been “stress relieved” for years, and all the “stresses” in a barrel have to add up to a total of “zero” as the barrel isn’t moving. So all the “stresses” in a barrel at rest equal out.
So, what does cause the “group opening up” and “wandering zero” problems? Heat, fouling, and non-uniform dimensions. But first, some background material.
The first thing you need to know is that every barrel has it’s own harmonics, and these cause the muzzle to whip about as it vibrates during the firing cycle. Changes to barrel time cause the bullet to leave the muzzle at different points in the harmonic cycle, which causes group size to increase.
The second thing you need to know is that there are many things that cause changes in barrel time. Load inconsistencies (bullet weight or geometric dimensions, powder charge, primer power levels) are one that don’t involve the barrel at all, but it should go without saying that shooting inconsistent ammo will give you inconsistent results.
But, with those two pieces of background information, on to heat, fouling, and non-uniform dimensions.
Heat causes metal expansion and strength changes. This doesn’t make the bore of the barrel larger, as the point of expansion is in the center of any given piece of steel, which in the case of a barrel is between the interior bore and the exterior surface. The exterior gets larger, the interior gets squished smaller. This causes a change in the force needed to get the bullet through the barrel and therefore changes the internal ballistics enough that it can affect the exterior ballistics.
Heat also causes the strength of the barrel steel to change to a “softer” or less strong state. A cyclic rate of fire can cause a barrel to fail on an M16 or M240B, so at some point in that heating cycle the barrel steel got hot enough to no longer be able to contain the pressure of a cartridge in the act of discharging and the steel offered less resistance to pressure than the bore, and “pop” goes the side of the barrel. For normal civilian use, this isn’t a concern as the temperatures are in the much lower end of the steel strength scale and therefore much less loss of strength. But even at the low end this can change the “tuning” of the barrel, as if you take three one inch rods of the same length but different materials, you would get a different sound when you struck each to listen for the resonant tone. This is why a metal vibraphone and a wooden xylophone sound different, as the different materials resonate distinctly.
Fouling causes an increase in friction, which causes changes in barrel timing and bullet velocity. Copper fouling and powder fouling both decrease the effective bore dimension and change the coefficient of friction. Because metal is generally “more slick” between two dissimilar metals, a layer of jacket fouling in the bore will be the most friction scenario for your rifle barrel. This causes a change in barrel timing, which causes a change in barrel harmonics, causing the bullet to exit the muzzle at a different point in the vibration.
Non-uniform barrel dimensions, especially in terms of concentric bore to external barrel and an out of true receiver fit, cause actual “stresses” or forces to appear as metal expands at different rates in the barrel (thin pieces heat through more quickly than thick pieces) and an out of true barrel to receiver mating will cause a moment effect as the steel pressure increases more on one side than the other.
Now, if you add all of this together, heat causing the bore to constrict which compounds the fouling problem, while the steel changes slightly in resonant pitch, and any non-uniform dimensions or untrue mating surfaces heat up to distort the barrel, you have a full explanation of the “wandering zero” or “shifting groups” problems often associated with thin rifle barrels.
So, how do you get a thin rifle barrel that doesn’t have these problems?
Stress relieve the barrel as normal during manufacturing. Machine the final exterior contour true to the bore so that there are uniform dimensions. Use a rifling profile that minimizes fouling (3r, 5r, or Polygonal for example). And finally install the barrel into the rifle so that all the mating surfaces are true. If you do that, then you should have a rifle with a lightweight barrel that holds zero even when shot hot.
Other things to consider.
Having a uniform barrel on a blueprinted action will do you no good if you have a stock design that limits barrel cooling on one side. Traditional stock designs suck at this, as the forend blocks a lot of radiative and convective cooling of the underside of the barrel even if the barrel is free floated. A 20 degree difference over 12 inches of barrel steel is going to produce between 1/3 and 2/3 MOA of deflection from normal, and also mess with the barrel harmonics something fierce. Most barrel harmonic simulators involve steel of uniform properties (heat, strength) and are mathematically perfect (everything is uniform), so anything that isn’t, is going to go well into the uncharted territories of “Well, I’ve got no clue what will actually happen to your groups.”
This is why a lot of lightweight hunting rifles don’t shoot well with a free floated barrel, and instead use a “pressure pad” at the front of the forend to provide a tuning point the same way a guitarist uses their finger to change the pitch of a string as it is plucked. Essentially the pressure pad raises the pitch, and the conservation of energy tells us that since amplitude will be dampened and the frequency must increase, so you can get a lower muzzle deflection across a larger range of bullet barrel times and velocities.
If you have a rifle like an AR, or “Tube Gun” style bolt action rifle (popular in Palma and High Power) then you can easily wrap the barrel in a free float tube which gives much more uniform cooling for radiation and convection.
Now, if any of you are still clinging to the “internal stress” explanation, let me point out that cold hammer forged barrels are the most “stressed” of any barrel manufacturing process. And cold hammer forged barrels perform the same as traditional drilled bore barrels made using cut or button rifling when it comes to heat. After all, all those stresses in a CHF barrel still have to add up to zero, and if you apply uniform heat to the barrel then you should get uniform changes to all those stresses still adding up to “zero.” Even in a highly “stressed” CHF barrel the real culprit is heat, fouling, and inconsistent dimensions rather than some non-measurable “internal stress.” One of the great benefits of CHF manufacturing is that if the steel isn’t highly uniform structurally and chemically to begin with, you won’t even get a barrel at the end.
Comments are open for bitches, gripes, complaints, stories, anecdotes, jokes, and musings.
For More Reading:
https://www.shootingsoftware.com/ftp/Pressure%20Factors.pdf (pay attention to the effect of barrel temperature on bullet performance outlined here!)
https://wanderingthroughthenight.wordpress.com/2016/10/29/reloading-rifle-powder-temperature-sensitivity/ (something I wrote a while back, applies mainly to “clean cold bore” shots in a hunting rifle and longer strings for competition shooters).
http://firearmshistory.blogspot.com/2012/11/the-effect-of-temperature-on-ammunition.html (a brief introduction on environmental impacts on bullet flight)
http://www.dtic.mil/dtic/tr/fulltext/u2/404467.pdf (a very thorough explanation of how barrels heat unevenly from the breech forward)