Barnes Manual No. 4 quotes as Deerwarden states. My understanding is that copper bullets generally have less friction in the barrel as rather than have full contact with the lands "driving bands"are a function of bullet design. In effect grooves are cut into the bullet at right angles to the axis of the bullet to reduced the contact with the barrel therefore require more powder to drive them. Seems like a contradiction but I believe the bands can be more easily altered to achieve maximum velocity and retain BC than altering the overall length of the bearing surface of the bullet. However I am not an expert in these matters and rely on the experts advice
I do not load Barnes bullets but I do load GS Gustom bullets.
There is a detailed explanation on the GS Custom website of why their bullets are constructed in this way and the way in which the loads have been developed to take advantage of their characteristics. I load 308 and on their loading charts, which they state they are only safe with their bullets, you can compare their loads for a particular bullet weight, the starting load and the final maximum velocity of the bullet with those of conventional bullets on other loading charts which you may have. My limited experience of these comparisons shows higher starting loads and higher maximum velocities with the copper bullets.
GS Custom bullets are at pains to point out that their loading charts should only be used with their bullets as with any other bullets, due to their composition and structure these loads could be dangerous.
A rather long winded way of saying that Barnes are probably working along the same lines but IMHO there is a better explanation on the GS Custom website.
You can also phone Gerald , the man behind the bullet, and he is generally happy to discuss problems, ideas and the theory.
Regards
Bob