Here's a comparison of the ballistics of a 220 grain SMK fired from a 300 BLK and a 30-06. I used the 30-06 instead of a 308 because a 220SMK won't comfortably fit in a 308 short action rifle.. They do fit nicely in a 30-06 in a Rem 700 long action. Conditions are standard metro.
Cartridge ---------------- 300 BLK-----------------30-06
Rifle ----------------------- AR-15 -------------------Rem 700 LA
bullet-----------------------220SMK-----------------220SMK
Barrel-----------------------16 in --------------------22 in
MV---------------------------1025 fps*--------------2425 fps **
Velocity @ 100------------990 fps----------------2291f ps
Velocity @ 400------------908 fps ---------------1914 fps
Energy @100-------------479 ft-lb---------------2566 ft-lb
energy @ 400------------402 ft-lb----------------1790 ft lb
10mph wind @100------0.9 in--------------------0.6 in
10mph wind @400-----13.81in-----------------10.6 in
drop @ 100 ---------------16.9 in ------------------3.1 in
drop @ 400 --------------289.3 in-----------------55.1 in
* 1025 is not max but about optimum for stable subsonic performance
** 2425 is about a 90% max load with H4350 powder.
What's better? That depends on what you're trying to accomplish.
