Might be something else besides copper...
I have heard of barrel temp though.
Check it out, more at link this is just the intro..
http://findarticles.com/p/articles/mi_m0BQY/is_4_47/ai_71323936/ Unless you're a guaranteed one-shot hunter, barrel heat will affect your accuracy. With the right data, however, barrel thermodynamics can actually work in your favor.
Generally, four classes of conditions exist within a rifle receiver when firing numerous shots. The first condition occurs when several shots are fired very quickly -- little heat can move from the barrel into the receiver.
The second occurs when several shots are fired very slowly, and a lot of heat moves from the barrel into the receiver, but overall cooling allows all parts of the gun to approach ambient temperature prior to each subsequent shot.
In the third condition, the first few rounds are fired in an ambient-temperature gun, which causes an intermediate rate of fire to exist in which the front-to-rear temperature gradient within the receiver becomes significant.
The fourth condition occurs during extended shot strings, at any particular (relatively constant) cadence, when the system eventually reaches equilibrium, so that, prior to each shot, both temperature and temperature gradient are essentially similar.
***
The only reason I post this, is my own account with my 7.65mm Mauser Bolt Action. After running through a couple of strings, 2 5rd stripper clips, the barrel is HOT when fired quickly.
My groups also "open" up not much but it is there. Wait for m25 , Ben will know more.
Just my .02 cents..