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.
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.
