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Discussion Starter · #1 ·
When you fire a rifle, the chemical energy stored in the solid powder is transformed into heat energy in the propellant gases. The propellant gases push against the rear of the bullet hard enough to accelerate it down the barrel. But not all of the energy can be used; some of it leaving as heat through the barrel. About 10 percent of the total heat energy may go into the barrel of small arms, 30 percent in the case of cannon.

In the case of rifles, the heat of the propellant gases that goes into the barrel depends partly upon the temperature of the barrel. While 20 percent of the heat may go into a cold barrel, only 5 percent may go into a hot barrel.

More importantly than the percentage of heat transferred to the barrel is the way in which that heat is distributed. If, after firing several shots in rapid succession, we touch our rifle barrel, we may say something intellegent like, "Ooo, it's hot!" What we can't see is that the inside of the barrel is far hotter at this moment. While our finger is fast imitating bacon on the outside of the barrel, most of the temperature rise will be within the first 1/100 of an inch of the bore surface. Our finger may soon be touching a 160 degree F outer surface, while the inner surface cools from over 1600 degrees F for roughly 5/1000 of a second afterwards. In that first 1/1000 of a second after several rapid shots, the inner surface will be hotter than a roaring fireplace.

With these temperatures, it seems obvious that bad stuff is happening to the inner surface of the barrel. Recall from high school physics that all circles expand when heated, and viewed from the end, a barrel is a circle. One problem is that the hot inner surface wants to expand far more than the cooler outer surface. This creates severe stresses in the metal that may lead to cracking, then erosion, on the inner surface. Another problem is that the barrel's inner surface may soften at these high temperatures, quickly wearing away. Water cooling may help, but it can't do much to stop the damage, as it's the inner surface of the barrel that suffers the most. The use of a barrel material that is chemically stable at higher temperatures will allow it to retain its hardness. Heat can also affect the harmonics of the barrel. In practice, most cold shots impact slightly lower, and a smidgeon to one side, than shots fired after heating.

Even if the barrel expands, the pressure of the gases will still push on the bullet's base hard enough to squish the bullet into the rifling. However, heat can distort the barrel, resulting in inaccuracy. Uneven heating of the barrel, as well as uneven stress distribution, usually causes a shift in the impact of subsequent shots.

Hunting rifles are typically worse about shifting their POI's than are military rifles. Modern military rifles shoot smaller cartridges than most hunting rifles, and the military rifles are designed with better heat management. Equipped with a scope, many modern deer rifles weigh less than a loaded, iron sighted M-16. Add to that the fact that one shot of a popular deer rifle cartridge may transfer more heat to the barrel than two or three shots of 5.56x45mm M-16 cartridges. Then look at the economics of rifles. The military pays extra for heat management features and "high hot hardness" materials on their rifles, while the average deer hunter coundn't care less about heat dissipation. The average deer hunter cares only about where the first shot from a cold barrel will land.

For deer rifles, the best way to avoid heat-induced inaccuracies is to fire a sight-in shot, then wait while the rifle cools. When you can hold the barrel with your bare hand (please don't touch it too soon), fire another shot and repeat. Measure your group, adjust your sights if necessary, then do it over again.
 

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Interesting post GB. I always wondered how heat affects a rifle barrel though I've always been careful not to let mine get too hot. Thanks

RIKA
 

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In the case of rifles, the heat of the propellant gases that goes into the barrel depends partly upon the temperature of the barrel. While 20 percent of the heat may go into a cold barrel, only 5 percent may go into a hot barrel.
This quote caught my eye. It implies to me that a hot rifle barrel makes more efficient use of the propellant gases then a cold barrel does. Meaning what? Do you get more velocity out of a hotter barrel?
 

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Discussion Starter · #4 ·
Rich,

Heat is energy. Heat flows from hot to cold. If the barrel and the propellant gas were at the same temperature, heat would not flow between them. Following this logic, if no heat left the propellant gas through the barrel, more energy would go into pushing the bullet. So, yes the hotter barrel would be more efficient than the cold barrel.
 

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This could probably evolve into a very interesting discussion! :)

Discussions about efficiency of powder burning, heat/energy transfer, and the effects of barrel material affecting velocity of the projectile.

Since much of the barrel heating is caused by the friction of the projectile, this would be another variable within the mix.

Of course, the difference would probably just be hair splitting in action, I would guess.
 

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Rolin:
Thwere is an out-of-print book named "The Principles and Practices of Reloading Ammunition, authored by Earl Narramore.

There are extensive passages on what you have posted.

Though out of print, the book can be readily found at the better gunshows.
 

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Discussion Starter · #7 ·
Thanks, Bill, I will try to find a copy.

Rich, the friction between the bullet and the barrel accounts for only about two or two-and-a-half percent of the total energy. The real culprit causing the barrel heating is the hot gas swirling against the bore, just like a steam radiator in an old building.
 

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Yeah, that seems logical to me. However although I may have been imagining it, it seemed to me that barrels where I was shooting moly coated bullets took a LOT longer to heat up then ones where I was firing regular copper clad bullets. Nothing more scientific then the "hand on the barrel" test, however. :)
 

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I beleive that some of the carbon fiber barrels (very thin metal tube with the rifling) surounded by a carbon fiber barrel to provide strength, are supposedly more efficient at dealing with the heat.

I may be wrong though, but that's one of the benefits, in addition to being lighter than a conventional barrel.

:devil:
 

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Discussion Starter · #10 ·
Aslan, I've heard that, although I've never calculated the the heat transfer on a carbon fiber barrel.

As a general rule...if a material conducts electricity well, it will conduct heat well, too. There are some exceptions, notably stainless steel and alumina.

Stainless steel conducts electricity just fine, but not heat. Stainless steel can be heated like carbon steel, but it doen't spread out the heat. You can hold one end of a barrel-sized stainless steel rod in your hand while holding a torch to the other end and not get burned. Try that with copper..ha ha. Alumina conducts heat just fine, but not electricity. As a barrel material, stainless steel can have lower friction and a longer life.
 
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