I want to pick a 6.5mm copper mono for elk and I'd like to stay under 1.42" length for 1.5 Sg in 1:9 twist in 264 Win Mag at high-altitude (6000 to 9000 feet). So that is the pre-determined criteria: .264", 1:9 twist, copper mono. I'm not considering anything outside those parameters.
There appears to be two designs: copper expanding bullets (TTSX, LRX, CX, e-Tip) and copper fragmenting bullets (Controlled Chaos, CEB, Hammer).
Because 6.5mm is regarded as marginal on elk and I'm necessarily choosing lighter bullets due to my twist rate's length limitations, I'm inclined toward the expanding bullets that retain the weight. Perhaps the shank of the fragmenting bullets maintains a sectional density equivalent to the heavier but expanded projectile. I don't know how that translates to actual results.
I see that Hammer claims on their website that their bullets need 1.5 Sg calculated at sea-level for effective terminal performance even if they're used at higher altitude. So, without more spin, does the bullet yaw and fail to open or the shank yaw and fail to exit? It looks like their adamant their bullets won't perform in my twist rate, so I'm disinclined towards the fragmenting type.
I'm inclined toward the Barnes 127 grain LRX because it has a shorter length (1.402") that will comfortably meet Miller-formula Sg of 1.5 at my altitude (I live at 5000' but the game is even higher), even in sub-zero temps, and because it's a proven-performer on elk. I already have them on back-order, but could cancel.