Mark,
I'll give this a little more thought based on some simplified mathematical analysis. I can state right now that as barrel diameter increases, steel barrel volume/mass increases to the 2nd power of barrel diameter (a squared function). Whereas exterior barrel surface area only increases to the 1st power - in other words a direct 1:1 increase of surface area due to increasing barrel diameter. So increasing barrel diameter will increase the mass of steel in the barrel at a greater rate than it will increase exterior surface area.

I'll update this post when I have some more time on my hands. But I think the increased cooling rate is minimal. The greatest advantage, with respect to heat up and cool off the larger diameter barrel has over the smaller diameter barrel is the larger barrel will take more shots to reach an equal temperature during the initial shot string, because of the greater thermal mass (heat sink effect) of the extra steel in the heavier barrel. And for the initial repetitive-fire shot string, a barrel that heats up more slowly will be less prone to warp or shift points of impact, compared to the thin-walled tubes.

There are, of course, other advantages to the heavier barrel, such as the additional mass results in less shifting of POA during the firing process. It takes more force to initiate motion of the larger mass than the smaller mass. But you've focused on rates or heat-up, cool-off, and even more specifically, heavy barrel versus light barrel temperatures under steady, methodical, sustained rate of fire. I believe that is really what you've focused on. For a given, sustained rate of fire over extended periods of time (like 2 hours), will the heavier profile barrel reach a higher or lower temperature than a lighter profile barrel.

My current perspective is this: the benefit of the heavier barrel under such sustained fire is substantially less than the benefit of the slowed rate of heat-up during the first string of firing.

Just to be clear --- rifles built correctly move very little if any when they get hot. The key is correctly. A factory tube with a 20 - 25 dollar tube on it is going to be very susceptible to movement as a result of heat—also the action stress will become apparent as well as leaving the ammunition in a hot chamber----I could go on but why you get the point.

I have had to shoot in matches that got the barrel according to my laser thermometer over 200 degrees.

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Mark,
I'll give this a little more thought based on some simplified mathematical analysis. I can state right now that as barrel diameter increases, steel barrel volume/mass increases to the 2nd power of barrel diameter (a squared function). Whereas exterior barrel surface area only increases to the 1st power - in other words a direct 1:1 increase of surface area due to increasing barrel diameter. So increasing barrel diameter will increase the mass of steel in the barrel at a greater rate than it will increase exterior surface area.

Paul I agree with your math, but the mass or volume is inconsequential to the question. What we are comparing is inflow to out flow and not how big the tank is. If we are pouring water into two water tanks with valves in the bottom of them and one valve is fully open and the other valve is only half open, we will be able to pour a higher rate of water into the tank with the valve fully open without over flowing it.

In the case of the barrels, the surface area is the valve and the surface is dictated by the mass (or volume) of the barrel. But the fact is that the bigger barrel as the bigger outflow valve allowing a greater amount of heat to be dumped into it without spilling over.

Bottom line is that rate you can input without overflowing is dictated by the output rate.

Quote:

My current perspective is this: the benefit of the heavier barrel under such sustained fire is substantially less than the benefit of the slowed rate of heat-up during the first string of firing.

I would say the benefit is directly proportional to the differences in surface area. With a Sendero and Finnlight, you could increase the rate of fire of the Sendero by about 25%. The advantage of the heavier barrel is that if you maintained that rate of fire, the barrel would remain cooler because it has more mass to absorb the heat. If you went to a heavier barrel with twice the surface area, you could double the rate of fire.

Last edited by MontanaRifleman; 05-03-2010 at 05:22 PM.

Metal is metal the mass is the mass--the bigger tubes heat slower and cool slower.

Boss, I think your assumption is that cooling is equal to heating. But I dont believe that's the case. They are two different processes dependant on separate conditions and variables.

The damage to a barrel is in the throat from excess shooting (heat, erosion). The throat (the inside of the barrel) will heat up faster and to a higher temp than the outside in the same time frame. If the outside of the barrel reaches 200 degrees as in Boss Hoss's post above, then the temp inside the barrel was higher than that at one point. The thinner barrel will heat up on the outside faster and give an indication of heat faster than will the thicker barrel. The inside will heat up faster than the heat can dissapate to the outside. Once heated a thicker barrel will take longer to cool than a thinner barrel

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Bottom line is that rate you can input without overflowing is dictated by the output rate.

Mark,
The reason I mention the mass factor is because the increased mass does affect the rate of heat (energy) transfer. The rate of heat transfer is directly dependent upon the temperature of the two bodies exchanging energy. In our case study, the temperature of the steel barrel and the temperature of the surrounding air. The greater the difference in temperatures, the greater the rate of heat transfer. The lower the difference in temperatures, the lower the rate of heat transfer.

The reason the mass of steel comes into play is it directly affects the temperature of the steel barrel over a given period of time. The longer the heavy steel barrel remains at a higher temperature relative to air temperature, then the greater the rate of heat transfer to the surrounding air. The faster the lighter barrel, with less steel mass, drops to a lower temperature - the lower the rate of heat transfer to the surrounding air.

The premise that "the mass or volume (of steel) is inconsequential to the question" is incorrect. Any property that results in the barrel maintaining a higher or lower temperature over any given period of time, "directly affects the rate of heat transfer throughout that given period of time". Since the mass of steel affects the temperature of any barrel over time, the mass of steel in the barrel is a contributing factor affecting the rate of heat transfer. Inflow and outflow are expressed as units of energy transfer per unit of time. The mass of steel does influence the rate of energy transfer, because it alters the temperature of the barrel over time.

To clarify your specific interest, is it correct to conclude that what you really want to know is this: Will the lighter or heavier profile barrel reach the higher temperature over a sustained, steady rate of fire over a prolonged period of time?

Increase barrel diameter from 11/16" to 1" diameter (plain straight cylinder shape), and the surface area increases 45%, for any equivalent barrel length.

With a .284 bore, the same increase in barrel diameter will increase barrel mass by 155%, for any equivalent barrel length.

I suspect barrel temperatures would remain lower in the heavier barrel under most reasonable scenarios of prolonged fire, as much due to increased barrel mass as due to any increased surface area.

At high rates of sustained fire, I expect both bores would be toast in short order, with respect to accuracy - no matter the diameter or the exterior surface temperature. The heavier barrel would probably dissipate the heat of several extra shots within its extra mass before it was also cooked.

When I consider these type of questions, I go to extremes, and that usually leads to insights which are reasonably correct. For example, what would be expected from a 12" diameter barrel versus a 3/8" diameter barrel. The exterior of the 12" diameter barrel will remain cool for a long time. But the interior bore surface could still be cooked with overly agressive rates of fire.