300 BLK has an advantage over 308 Win when subsonics are used and that is TWIST RATE. Unless one is ordering a custom barrel for 308 with a twist of 1:8" or faster, the 300 BLK is superior for subsonic use. Accuracy difference is moot. Neither is benchrest accurate with subs, but 300 BLK can fire projectiles with more mass (generally, due to twist rate) and is less expensive to load.
I have used both 300 BLK and 308 for subs with a suppressor, and a 1:10" twist 308 barrel will not stabilize heavy projectiles. Heaviest I could go with 1:10" subsonic was 185 grains. 190's keyhole. 300 BLK barrels are 8" twist or faster. My 1:8" works for up to 230, and my 1:7" barrel can use the 240 SMK.
For those that ask why 300 BLK over a subsonic 45: VERSATILITY and SECTIONAL DENSITY.
While a 45 acp can do subsonic as well as a 300 BLK, it's a one trick pony. It can't also fire 110-125 grain bullets at 2350. Nor does it have the sectional density for penetration or resistance to wind that heavy 308 projectiles do, so there is an advanatage there.
To stay on topic: this was a natural move for Ruger. I don't care for the Mini-14 and won't be buying one but it should sell for them, and it helps their partner Hornady sell more ammo so I think it's a good thing in the market place.
I'm amazed at how fast the 300 BLK has taken off, and I think it's for good reason. I don't always require the power or reach of a full power 308 to hunt, I like being able to take a 6 lb AR and go.
