You have to actually change dimensions to cause work hardening, at least in the elastic range if not permanent deformation. The best you could do with tumbling or even shot peening is a minor change in surface hardness, which has little to no effect on bullet expansion, and only if you did it enough to dimple the surface. That does nothing for hardening the inside of a copper part.
If you want to harden copper you need to actually work (i.e. deform) the material. Hammer it, squeeze it, form it, etc. I was wondering if you had made a swage setup of some sort to handle solid copper.
Barnes does not "harden" their bullets for different applications, although it's possible they anneal some or all of them after forming. They tune for different velocity ranges by controlling the size and shape of the hollow point and petals.
Last edited by Yondering; 12/15/17.
