friction seems like a much more likely source
The numbers just don't add up. The highest reported bullet drag force I've seen was in an Army report claiming 400 lb for a .30 conventional bullet (original X bullets were much higher). Over a 2 ft barrel, that's 800 ft-lb total drag energy which works out to 1 BTU.
A 3 lb barrel needs about .36 BTU for each 1 F of temperature, so we're looking at a 3 F rise from bullet drag, and that's the worst case numbers I could find--and it assumes all the friction heat goes to the barrel with none to the bullet. Other sources give a far lower average bullet drag force.
The 7.62 NATO releases about 11 BTU of chemical energy. Hatcher's notebook has some old military data suggesting 22% of the gas energy goes to barrel heating and 7% to bullet friction. Now, 7% of 11 BTU is .77 BTU, about in line with the 1 BTU figured above.
My notes don't record if Hatcher's 22% for barrel heating includes or excludes the 7% for bullet friction. The table I recorded suggests it is excluded, so his numbers imply convective and radiative heat transfer from the gases to the barrel are triple the heat energy of bullet friction.
