The 6.5 is fine in a shorter barrel. Brothers shoots a 147 a 2600 just fine. In terms of energy it maintains it much longer. A 18 inch 308 shooting 178s will still be losing velocity vs a . The 6.5 creedmoor is surprisingly efficient due to the shorter case especially with heavier bullets. Ie read this
https://rifleshooter.com/2016/02/6-5...p-a-creedmoor/ and this
https://rifleshooter.com/2014/12/308...ty-28-to-16-5/ notice the less lose per inch on 140 6.5.
Just inputting in quickload shows 2675fps should be possible for a 147 eld m at the top end on the creedmoor where as 2610 for the 178 (eld m for comparison purposes) in the 308. (I don't have the latest powders so potentially you'd gain a bit more in both but unlikely the 308 would as it seems the new rl16 and rl26 etc suit the creed more.
To compare the above bcs you are getting down to that 1800fps at 500yard with 308 which has about 1300ftlbs. Which is heaps to kill and anything before this is dead as dead really. You can push the 308 to 650 where it now has 1600fps and 1000ftlbs but you are on the envelope of the bullet not expanding.
The 6.5 drops below 1800fps at 700 Yards with just over 1000ftlbs.
Do realistically inside 300 they both kill dead everything as will almost all 6.5 and up 308 based cases. At that point energy is almost equal but the creed starts to creep away. Also the creed has more velocity to reach that energy as such it is more likely to expand and use the energy.
They will both work at any range your considering but the creed does it better beyond and with less recoil and wind drift at range. The 308 does have more diameter but I'm not a big believer in that argument as once expanded it a much a muchness. The 308 I find doesn't have the same amount of quality factory options and the high bc bullets don't fit the magazine length great when you do reload.
After all that favouring the 6.5. I'll note I used a 16.5" 308 bergara for everything (mostly tops for the last 2 years. Very nice trigger for what was a 700 gun brand new. I shot the factory hornady superformance match 168 eldm as I couldn't get close to that velocity with reloads and have accuracy 2670fps. With this I took a chamois at 600 yards and also my brother took a red hind with my rifle at 650 yards. I also used the 225 eld Ms at 2350fps which I took a chamois at 450 yards. The 308 is more than capable at realistic ranges especially as with elevation your bullets maintain velocity much better as such increasing your range significantly.
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