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As anyone who's done some shooting quickly finds out, it does not take very many rounds to heat up a barrel to be too hot to touch.

A lot of folks think the 'burning of the propellant, and its subsequent increase in temperature, is entirely what provides the pressure, and so it would make sense that the propellant alone provides all that heat. However, I don't think that's the whole story, as much of the pressure isn't because of the rise in temperature, but because the propellant's large molecules break down into numerous combustion products, and with most (in the case of smokeless powder) of those products being gaseous. A near instant transition from solid to gas generates most of the pressure, so if a propellant somehow burned 'cold' I think it would likely still be quite effective.

Additionally, it takes a surprisingly large amount of force to push a bullet down a barrel. Anyone who's had to clear a squib load can attest that you really have to bang on it to get it out. Some material I've read indicates forces as high as 300 lbf needed to push a bullet along the barrel. There is the friction of the barrel with the sides of the barrel, along with the force needed to deform the bullet into the grooves of the rifling.

So, for a typical long-gun, which effect transfers the most heat to the barrel? Heat from the burnt propellant, or frictional and deformation effects of the bullet traveling through the barrel?

If we want to compare a specific combination, let's say an AR-15 type rifle with a typical 20" barrel and 5.56 ammunition.

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I propose an experiment in which you shoot off a series of blanks and measure the temperature rise of the barrel. after allowing the barrel to cool, repeat with live loads and compare the result.

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  • $\begingroup$ Would the hot gases spend the same time confined to the barrel with no round to slow them down? $\endgroup$ – DJohnM Dec 20 '17 at 23:31
  • $\begingroup$ good question. I don't know! but perhaps I can think up another experiment ;-) $\endgroup$ – niels nielsen Dec 21 '17 at 0:18
  • $\begingroup$ @nielsnielsen: while that would be a fun experiment, I don't have the equipment to do that laying around nor the space to conveniently perform it. Additionally this is a non-answer. $\endgroup$ – whatsisname Dec 21 '17 at 5:14
  • $\begingroup$ then let's wait and see if anyone else can answer your question properly. $\endgroup$ – niels nielsen Dec 21 '17 at 5:16

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