Comparing the above with other information from a police department test of an M14/M1A, data from Battlefield Vegas, as well as information from the Lucky Gunner test, I've drawn some additional conclusions as well as one that contradicts Lysander's conclusion from the above:
If usable accuracy, not velocity loss, is the prime concern, especially with high rates of fire using boat tail bullets (FMJBT, OTM, etc), then copper jacketed bullets are a far superior choice and will result in 2x - 4x the usable barrel life compared to bimetal jackets, regardless of any other factors including barrel construction. Bi-metal bullet jackets cause a different, more destructive type of wear that strips the rifling from the barrel all down the length of the bore. Powder burn temperature remains a significant factor in barrel life, bi-metal jackets will simply kill a barrel in a different way before powder differences become significant.
How different bullet jacket materials affect barrel wear with slower firing schedules isn't clear. It has been suggested that steel jackets raise barrel temperatures faster than copper due to differences in friction. What is clear is that, at high rates of fire, steel jackets wear the rifling all down the bore's length, not just starting from the throat and creeping forward as is seen with copper jackets.
AKs in 7.62x39 are known to fire accurately with bi-metal jacketed ammunition far in excess of the 5-7k rounds that caused keyholing in the AR15 barrels used in the Lucky Gunner test. There is evidence suggesting the reason for this is that most types of 7.62x39 ball use flat base (non boat tail) bullets. Flat based projectiles are often recommended by experienced reloaders when attempting to wring the last bit of useful life from a worn barrel. It's not that flat based bullets cause less wear, it appears that they simply perform better in worn barrels than boat tailed bullets do.
So, if you have a 7.62x39 firearm that you've shot extensively with bi-metal jacketed bullets and it appears to have lost its accuracy in only a few thousand rounds, check your ammunition to see if it is using boat tailed or flat based bullets. If it won't shoot with flat based bullets, you can safely assume it's time to replace the barrel.