What I heard is the groves in Barnes, GMX, Hammers are to reduce bearing surface which aids in relieving some pressure and helps reduce fouling . Haven't heard anything about the crush affect but could be true . I don't recall exactly what the groves in the E Tips looked like . Minimum I guessing .Everyone seems to be right on with my experience also. I’ve has great luck with Barnes (TSX, TTSX, and LRX), so-so with Hornady GMX, and busted with Nosler E-Tips. As others have noted, I learned early on that copper bullets are longer than lead, meaning that you may have to get a lighter (ie shorter) bullet to match your twist rate.
One thing I haven’t seen stated here yet is that copper/guilding metal bullets are also typically HARDER than c&c bullets. That’s why they need the relief cuts in order to smash into the rifling properly. One effect of that hardness, coupled with the longer bearing surface in n the barrel, is that you can build excessive pressures very easily, especially with a fast-burn powder. Therefore, it’s very important to use load data specific to that bullet, or at least for monolithic bullets. You can’t take your old recipe for c&c bullets and just stick a copper in there of the same weight. You’ll probably have to change your charge. That pressure difference may also be why so many have noticed that monos like that free bore jump.
One more plus for Barnes is that their load data is typically freely available on their website. I’d start there.