Where does the heat come from?

Rocky;

I'm certainly not a physicist either, but in thinking about your question, I'd guess 80% or more of the heat transfered to the barrel is from the burning of the powder rather than the friction.  Consider that the pressure rise from ambient to 60,000 psi happens in a time frame measured in milliseconds over a distance of around 22", to pick some believable numbers.  I'm told that the gas exiting the muzzle is as solid as steel & hot enough to cut that steel like a hot knife through butter were it a sustained process.  Else why do throats erode?

But it is an interesting question.

900F

Friction is a big factor. I read in Varmint Hunter that 30 percent of the powder's energy goes into overcoming bore friction. And there is an energy transfer from the bullet/rifling thing...you're increasing the rate of spin while sliding along...you can't tell me that there isn't a significant amount of heat from that.

I think that about 40 percent of powder energy actually goes to moving the bullet and the other 30 percent gets blown out the dumb end.

Well I'm no physicist either but I know enough to get myself in trouble and I'll try my best to do just that

Pressure = Force / Area
Friction = a friction coefficient number X normal force (a.k.a. weight, mass X gravity)
Kinetic Energy = 1/2 mass X velocity squared

Heat is a measure of kinetic energy.

I'd most certainly say the pressure is a larger culprit than friction when it comes to generating heat for firearms.  The heat of the combusting gasses will be determined by a high pressures and a large surface area (the internal area of the barrel).  Whereas friction will be merely determined by a friction coefficient and the mass of the bullet.  Both of which are smaller in comparison to the components of pressure. 

I ain't no physicist but I can do algebra and knowing which numbers are bigger before plugging them into the equation makes sense to me that the heat generated by pressure makes more sense.  Maybe there's some engineer in here who can talk about combustion physics but it ain't me!

Interesting number crunching though.