I had an interesting question from one of my readers. It was well thought out and it expressed a concerned about the thin profile barrels used on the Kalasnikov rifles.
My response is somewhat lengthy and at times technical but I think it captures the essence of what I was trying to communicate to the gentleman. I'd like to hear your thoughts.
A discussion of one of several, perceived, weaknesses in the Kalashnikov design – the “pencil” barrel – needs to take place because it’s paid a ton of lip service without any real substance or objective discussion.
In designing barrel profiles, we need to consider a number of factors: pressure, temperature, length needed to stabilize the round, weight and sustained rate of fire. All of these contribute to the designers thought process. Other noteworthy considerations are heat conductivity and axial sheer, which is imparted to the barrel from the effects of rifling on the projectile as it moves along the bore. I’ll begin with heat conductivity because it is normally acknowledged that heat has a deleterious effect on performance.
Heat is transferred from the bore to the outside of the barrel. The faster it conducts to the outer surface of the barrel, the cooler the bore temperature.
Cooler temperatures mean that bore imperfections and rifling will have less of an impact on the bullet as it travels through the barrel, and accuracy should benefit.
It follows that faster heat transfer is desirable in order to keep the bore temperatures down.
The equation above is derived from Fourier’s law of heat conduction and succinctly describes the elements involved in determining heat transfer rates.
q = heat transfer rate
L = wall thickness of the heat conductor – in this case the barrel.
k = the heat conductivity constant of the material used (more on this lat*er in the article)
A = the cross-sectional area of the conductor.
?T=The temperature gradient
(difference between Temperature IN and Temperature Out)
Without running elaborate measurements and calculations, simple evaluation of the equation tells us that a smaller L (wall thickness) gives us a higher q (heat transfer rate). Therefore, a thin barrel, all things being equal, will transfer heat faster.
Heat transfer problems occur when we encounter conductivity barriers – for example steel to air heat transfers. On a rifle barrel, the problem occurs on the outside surface – going from steel to the surrounding atmosphere.
Because carbon steel has a heat conductivity constant of 43 and air’s is .024, it takes a much longer time for heat to conduct to air, irrespective of barrel wall thickness. The end result is ?T becomes smaller and the barrel’s heat transfer rate slows. When ?T is zero, heat transfer bore to outside surface stops. This is a situation that must be prevented.
Fluting or dimples reduce barrel wall thickness and increase exterior surface area acting like cooling fins. They’re a highly effective way of dealing with the heat transfer problem, especially in applications where a sustained high rate of fire is anticipated.
To summarize, the take-away from this discussion is that thin barrels are not a bad thing – especially true if you are humping the Hindu Kush with 100+ pounds of other stuff.
The other component of barrel profile that I want to touch on is structural, and has to do with axil sheers.
A chambered round with a locked bolt is a closed system. When powder ignites, gases of sufficient magnitude and pressure are produced to push the bullet through the bore. The bullet forms a gas seal with the rifling so little or no gas leaks around the bullet’s circumference.
Bore pressures will vary with caliber and ammunition but they are extremely high and range from 50,000+ psi to 20,000 psi. This sudden and forceful impulse acts along the length of the barrel. Furthermore, as the riffling applies a spin on the projectile, axial forces are exerted along the length of the barrel. Here, heavy barrel profiles can help to reduce, but not eliminate barrel harmonics that may or may not detract from accuracy. This is of concern with precision rifles, for example snipe platforms, not rifles or carbines intended for sustained high rates of fire – with some exceptions like a light machine gun (LMG)
The last two pages are what writers refer to as support, but the idea I’m presenting is that the common barrel profiles found on the Kalashnikov rifles are not poor limiting designs, but rather a balance struck that takes a number of factors into consideration.
It is technical, but explanation makes sense. All has been considered in the barrel design