6.5 CM with a 140-156gr bullet. Recoil relates to the cartridge is primarily due to bullet weight and secondarily due to powder charge. If we are only considering modern centerfire cartridges, then the rest of the factors are pretty minimal. Powder charge in a .243, 6.5CM, .260, 7-08, .308 are all pretty much the same. If you’re trying to keep bullet weight reasonable so that you don’t hammer her shoulder you’d be looking at 140gr-156gr in a 6.5CM or .260, 140gr-168gr in a 7-08 or 155-168gr in a .308. If bullet construction is similar(say you’re going to shoot an Accubond out of all three, or a partition out of all three etc.) then penetration primarily comes down to sectional density. Sectional density of similar weight bullets will be highest with the smallest bore diameter. And thus, if one is going to shoot roughly the same weight bullet from all three cartridges, and use basically the same type of bullet, the 6.5CM or .260Rem is the most deadly. The only reason a 7-08 or 308 can be more deadly, is that you can conceivably shoot bullets far heavier than 155gr from both of them.(195 in 7mm and 240 in .308) Obviously if recoil is a concern, you don’t want to be shooting heavy bullets from a 7-08 or 308, so why bother with either one?
