Know that I’m late to the party here but just wanted to share my experience with 6.5mm cartridges/bullets over the past few years.
For decades I was a committed 300win/300wea 180gr guy. They killed everything from deer to brown bear. But they often weren’t the “flying sledgehammer” that legend/lore made them out to be. In addition, magnum action rifles usually aren’t that light, handy or easy shooting (especially in the field) as their smaller cartridge cousins. They also have considerably more blast/recoil — I’m not a fan of adding the weight/length of a suppressor to a hunting rig, and they aren’t legal everywhere.
Bought my first 6.5 back in 2016 as I was increasingly focused on mountain game. For sheep/goat hunts weight of everything is critical. Grams turn into ounces, ounces to pounds, and pounds into pain. In addition to the rifles, both 300 and 7mm ammo is considerably heavier than 6.5PRC. Right from the start I was impressed with the performance of the 6.5s.
Since then I’ve taken quite a few sheep/goats out to ~600 yards. The 6.5CM & PRC have truly impressed everything I’ve ever pointed them at. What’s really impressed me are both the damage and penetration on game, even at distance. Most shots were complete penetration and the few that weren’t were quartered toward me and hit heavy bone early in the wound channel. Used 140gr Accubonds and 143 ELD-X. Love them both. My confidence in 6.5s is based on repeated performance on game.
Now, will they be my choice for an elk hunt? My point totals in several western states are getting close to guaranteeing me a tag, so we’ll certainly find out. Do I believe a 6.5PRC is enough for a mature bull elk at reasonable hunting ranges? - YES. Would I feel more comfortable using a 7mm PRC (I have one), or a 300? The jury is still out - a lot will depend on how well I’m shooting individual rifles. But for great bears it will be a 300win bonded bullet at a minimum.
Another point is confidence in one’s equipment. I was a committed bow hunter for a good portion of my hunting career (and still hunt that way from time to time). There is just no substitute to the firm conviction that if I do my part the result will be a clean kill.