At either barrel length, Using similar weight bullets (the only real overlap is in the 150 grain range) the .308 will provide better muzzle velocity however; (and this is a big one) within about 200-300 yards the creedmoor is moving faster, will drop less and retains better energy down range for smacking steel or soft gushy animal materials.look up MV numbers from similar builds, pick a 6.5 and a.308 bullet of roughly the same weight and type, run the numbers for both in a ballistics calculator, and plot the drop, velocity and energy out to 1200 yards, the difference is shocking. I have been looking at this comparing 6.5 cm, 7-08, and .308 in 16, 18, and 20” barrel lengths, so far the sweet spot seems to be the 7-08 with 150-175 grain pills. All of these cartridges dip below reliable expansion velocity around 400-500 yards and drop below the mystical 1000ft-lb energy “requirement” that gun writers always prattle off with no real understanding in that same distance. The 1000 ft-lb number is probably arbitrary but the expansion velocity is not, you need big wound channels and hydrostatic shock to vitals to anchor animals. It seems that adding or cutting 2” of barrel will change MV enough to extend or shorten that “ethical”range by 75 yards or so in rough general terms.
Sure you can increase the downrange performance of the .308 with 180 grain bullet but the trade off is in increased recoil. Sure the .308 makes more efficient use of the available powder charge but the downrange advantage goes to the 6.5 and 7 mm bullets, and don’t get me wrong, I love a .308 or .30-06 and wouldn’t consider either obsolete, but the math doesn’t support it being the king when you factor in Recoil in a lightweight rifle or long range performance with a shorter barrel. in my eyes,recoil doesn’t matter all that much so I wouldn’t totally rule out an 18” .308, it’s still probably overkill for hunting out to 300 yards or so, and the reloading and factory options for it are staggering to meet special criteria, but I already have a 20”.308 so I can’t justify the expense of a slightly shorter version of it while still I’m kid-poor. If you have a long 6.5 already, this might be a nice addition to it for field use.
for me and my uses, running the numbers of several different bullets in several different barrel lengths shows an Optimal configuration of a 7-08 (maybe AI) running 150-162 gr. bullets from an 18” barrel. The 7 mm gives the option of considerably heavier bullets with great sectional density in case I need to hunt a bear and still gives mild recoil with ~150 gr. Bullets that nips at the heels of the 6.5 bullets at longer ranges. Add into that the longer barrel life that everybody worries so much about and the 18” 7mm is a nice middle ground thats still pretty short with a suppressor. I live in the south, shots rarely exceed 400 yards, if you live in a place with more sky and less trees, your solution may be different, but this seems to be a winner for a practical rifle that can do almost all of my hunting desires where I live and still play the target game for fun. I’m looking for a used Tikka locally to make this a reality. Just be honest with yourself about what you want the rifle to do and be analytical about what you need to meet those goals and that can guide You to a solution.
