Now we’ll look at an instance where the conventional wisdom is correct, but a little ambiguous. Most of us have heard that “sticky extraction and hard bolt lift” are early warning signs of excessive pressure in loaded ammunition and that sensible reloaders should back off a bit. The concepts are easy enough to understand and to recognize, but exactly what’s happening might not be. It’s just “sticky extraction means back off.” Let’s think for a moment about what happens when a cartridge is fired in a bolt action rifle and the mechanism of hard extraction. A brass cartridge case has two main functions. First, it’s a package, holding various components in a durable and waterproof unit. Without this package we wouldn’t have reliable repeating rifles, and the great armies of WWI would not have had machine guns to slaughter each other. Second, and not so obviously, the case is a seal. It’s designed to “fail,” to expand against the chamber wall on firing and to prevent propellant gases from escaping backwards around the outside of the cartridge. It’s a clever design; essentially the front of the case is a sturdy package up to the instant of firing, then it deforms easily to seal the front of the chamber, and all the gas pressure can be restrained by the surrounding steel barrel. The solid case head – much harder and stronger than the front of the case – is NOT intended to fail. When it does, we have a blown primer if we’re lucky, possibly much worse. The case head, unsupported by the barrel, is forced by gas pressure towards the rear and the shooter, and must be supported by the breeching mechanism. That’s the locked bolt and the load on it is around 5,000-10,000 pounds, pushing directly back towards the shooter’s face. That’s not a pressure, that’s a force and it’s equivalent to two or three cars parked on the bolt, which is the only thing keeping mayhem away from the shooter’s face. The force might be reduced a little by the case walls, depending on the case taper and lubrication, but not much.
Adding flutes to a rifle barrel makes the barrel lighter, more flexible, and has only a minor effect on cooling. Flutes can look nice though.
All solid materials – wood, concrete, glass, rubber, steel, Swiss cheese – have properties that engineers call a “modulus of elasticity,” and “elongation at the yield point.” In simple terms, the concept is that all materials are springy to a varying degree. It’s pretty clear that a rubber band will stretch a lot before it breaks, and that the stretch can be recovered if the rubber band is relaxed before it breaks. It’s equally true of steel and concrete, at least in engineering terms. They’re just not as stretchy.
What’s this got to do with hard extraction? When a rifle fires, the barrel swells radially and the receiver stretches. The part of the receiver ring surrounding the barrel gets larger in diameter, and the part surrounding the bolt locking lugs gets longer as gas pressure tries to push the bolt to the rear. At the same time, the front of the bolt is compressing, getting shorter. The chamber is getting both larger in diameter, and longer. Headspace is increasing. During firing the steel parts of the rifle are not stressed beyond the yield point. All the stretch is temporary and is recovered as soon as gas pressure is released – essentially instantly. This isn’t true of the brass cartridge case, at least in the body area. It has “failed,” stressed beyond the yield point and deformed permanently. This is obvious – otherwise it wouldn’t be necessary to resize the body before reloading.
Now steel barrel and receiver plus brass case all expanded together, but didn’t contract together. The brass contracted less than the steel, the amount determined by the maximum pressure. Normally, the difference is small, and the fired case is a tighter fit in the chamber than it was prior to firing, but it still comes out easily. If chamber pressure was high enough, the contracting steel parts squeezed the case firmly, and instead of enough clearance to allow extraction, we have a tapered case firmly pressed into the chamber. The higher the pressure, the tighter the fit, and the errant reloader may have to resort to brutality to extract the case.
This badly eroded "fire cracked" section of a rifle barrel shows more chmical attack than mechanical. Photo courtesy of Gradient Lens Corporation.
We can deal with the conventional wisdom surrounding fluted rifle barrels quickly. First is the all-too-common statement that fluting makes a barrel stiffer. Why anyone would think that removing material adds stiffness escapes me, but this is probably just a result of people not reading the fine print. What is true is that a fluted barrel will be stiffer than a similar barrel – of equal weight. This happens because the fluted barrel must be larger in diameter to be of equal weight, and increasing diameter adds stiffness quickly. The second often seen statement – that fluted barrels cool faster – is true, but only somewhere around the second decimal place. In still air no shooter will ever see a detectable difference. Why? It’s because the flutes run in the wrong direction and are ineffective for cooling. Look at a baseboard convector and notice that the flutes there are vertical, and perpendicular to the length of the hot water pipe they surround. That’s what properly designed convective cooling flutes look like. If you want your stainless barrel to cool faster, paint it black. That will have more cooling effect than flutes, as long as it is out of the sun anyway.
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