So to the OP's question, bullet/barrel friction only accounts for about 2% and heat transferred from the burning powder to the barrel is about 30% (almost as much energy as used to move the bullet!). Am I reading this right?
Not sure. The energy figure associated with friction is characterized as mechanical energy, not thermal energy. What did he mean?
I would have assumed that, in forcing a bullet of groove diameter down a rifled barrel, one would convert the energy used in deforming the bullet into thermal energy, heating both the bullet and barrel a great deal.
The effects may not be that great, however. Consider the operation of a milling machine. Both the cutter and the part do get hot as the cutter deforms the material beyond its shear limits, but a dribble of coolant prevents damage, and said coolant does not vaporize from the heat.
I have one nagging thought, however. It
seems to me that when I fired jacketed bullets in my Model 1903 rifle, the barrel seemed to heat up a lot more than when I fired softer bullets cast from linotype metal with rather similar propellant loads. However, that was a long, long time ago and I took no measurements.
The heat "
transferred from the burning powder" (as in holding a match under a knife blade for just a millisecond) may not really explain things either. To evaluate only the direct conversion of chemical energy to thermal energy, one could simply burn some smokeless propellant in an open container and measure the thermal energy output in calories.
There's also the pressure component. In a high powered rifle, the pressure rises to, say, 65,000 psi from a nominal 25 psi, due to the rapid introduction of gas as a byproduct of combustion, while the volume of the container expands only as the bullet moves down the barrel. As billybobjoe points out, the temperature would rise very considerably due to the gas pressure.
The only way to find out how much would be to know the volume of burned gasses created at ambient pressure (or the amount, in moles) and the gas constant and to calculate the temperature rise that would result from very quickly injecting said amount, starting with the gas at ambient temperature, into the barrel. Because the bullet moves down the barrel increasing the volume of the container, integral calculus would be needed.
I don't know the answer, but my
impression is that the barrel is probably heated much more by gas pressure than it is heated either directly from the flame or by friction.
