barrel heating in two of my rifles

[bluebayou]

I have a 7-08 Remington 700 that is very accurate and consistent.  I shoot 37.5 grains Varget behind a 160 gr Sierra Game King.  I also have a Winchester M70 in .270.  The load there is 52.5 grains of H4831 and 150 Hornady SP. 

If I shoot 5 rounds in the Remington it is pretty warm.  10 quick shots gets it hot enough to burn skin.  With the Winchester, I shot 20 times and the barrel was pretty warm.  I realize that this is subjective, but what is at work here?  Pressure?  Free bore?  Neither is a max load.  Is the Winchester barrel smoother?  Neither one really fouls up more than the other? 

Any ideas?  Not that it affects my shooting.  It has just been twisting inside my head that this is weird. 

[beemanbeme]

The only thing that comes to my mind (and it is indeed a WAG) is that the heavier bullet is staying in the barrel longer and allowing more heat to transfer to the barrel.  You would think however that the two would be about even considering the difference in calibre. 
What's your respective velocities? 
4831 is quite a bit slower than Varget.  Wonder what effect that would have?

[iiranger]

This is a "balancing" act. Basic physics. There is a certain amount of heat (BTUs) in the powder. It would seem since you are burning more powder in the .270 that it would get hotter, BUT how much "mass" of material are you heating. Don't suppose the 7/08 is a carbine with a short, light barrel? (Less mass?) And the .270 has a longer, heavier barrel? (More mass?)

My second question/experiment would be to fire after dark... 4831 is known for "slow burning." If you reach the spec pressure and the bullet is moving and leaves the muzzle and a quantity of burning powder exits the muzzle... That "heat" is lost into the "air." Does the '08 do a similar fireball?

These can be "fun" questions to ponder, but if the bullets are going into nice tight groups... that is what counts. luck

[Castaway]

The flash you see in dim light is not unburned powder, it is the super heated gasses exiting the barrel.  Powder, even slow powder like 4831 is still burned within an inch or so of the chamber.

[Catfish]

Castaway,
   If it is true that all of the powder is burned with an in. or 2 of the chamber why do you get higher velosities in longer barrels with slower burning powders? I have blown powder out the end of the barrel and found it in snow, some still unburned. You need to do some studing on internal blastics and presure curves.
   As for the barrel heating there are alot of thing that enter into barrel heating. 1st thing we think of amount of powder which is a big factor. 2nd. thing we usually think of is the mass of the barrel, hravy barrels take more to heat and radiate it off better. 3rd thing is outside temp. Barrels always heat faster on warmer days. 4th is rate of fire, if you fire faster the barrel will get hotter.  I am also convinced that that it takes more powder to heat barrels with larger bores. If you look at all of the thing you will see why one heats faster than the other.

[bluebayou]

Ambient air temperature might be an issue here.  Good point.  The 270 is shooting a lot more powder by volume, but it is slower.  The unburned powder idea is a good suggestion, but I would have to veto it in this case as I am not getting carbon marks blowing back on the case neck from low pressure or unburned powder/soot in the barrel.  I am pretty confident that it is burning.  The speed suggestion from Beemanbee kinda interests me.  The 7mm bullet is bigger and in front of a smaller powder charge so it is slower pretty much by default (I think).  Maybe it is a moment of inertia/pressure/heat transfer kinda thing going on.

Anyway thanks for the suggestions.  Going to borrow a chronograph and clock them.  Like I said, it just seems weird.

[Castaway]

Catfish, you get higher velocities in longer barrels because the pressure has longer to work on the bullet.  One reason you might be getting powder residue in your barrel is the resistance of the bullet to acceleration.  By that I mean; in a pistol cartridge, a heavy crimp or heavier bullet allows the powders to more fully ignite, in a rifle cartridge, a heavier bullet, seating the bullet touching the rifling or even a crimp will also offer resistance.  The powders still burn essentially at the chamber.